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Recommendations by SNC-Lavalin
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8.2. Recommendations by SNC-Lavalin
Our assessment of the aerobic stabilization process at the Kyiv Vodokanal wastewater treatment plant has lead to the following recommendations:
In the immediate future, continue using the aerobic stabilization process.
In the long term, when anaerobic digesters have been rehabilitated, surplus activated sludge should be jointly treated with the primary sludge by anaerobic digestion.
WWT-14 Monitoring Equipment
Background:
The plant is not automated and is operated on a manual basis. The only instrumentation available is the inlet flow measurement at the venturi flumes of each treatment line. The signals are used for flow indication and recording of total flow only.
All electrical and mechanical equipment is operated manually based on daily or weekly laboratory analyses of process parameters, primarily dissolved oxygen content and mixed liquor suspended solids in the aeration basins.
All analyses are made in the plant laboratory; no portable analyzing instruments are available. The tasks of sampling and analyses are time consuming and labour intensive.
COWI Recommendations for Improvement:
The following main components were recommended. A final list of monitoring equipment should be agreed to with management and operation staff at Bortnicheskaya WWTP.
- Portable oxygen meters;
- Stationary oxygen meter stations;
- Portable suspended solids meters;
- On-line ammonia meter;
- Flow meters;
- Level meters; and
- Training course in operation and maintenance of equipment.
Comments by Plant Staff:
During the site reconnaissance, interviews with staff resulted in the following comments:
- Both operations staff and laboratory personnel were interested in obtaining monitoring equipment.
- Operations staff acknowledged that adjustments to processes are slow, usually adhering to the following pattern:
- On-site laboratory staff takes samples (daily and weekly sets), analyzing samples on an agreed upon schedule of key parameters;
- Results are used to adjust process controls;
- Reports are sent to regulators on a monthly basis for their comments.
- Some operations personnel commented that the facility has very little control over the quality of wastewater it receives; and that the wastewater mainly contains municipal sewage, which is not subject to significant fluctuations in quality. The staff operating the facility are knowledgeable about making adjustments to the process based on years of experience.
SLE&C Assessment:
During the site reconnaissance, no monitoring devices were identified, whether stationary or portable.
Laboratory analytical results are generally not received in time to make adjusts for small upsets, only for general trends.
The installation of direct-reading monitoring equipment would allow for more immediate adjustments. In particular, the use of portable or on-line measurements of dissolved oxygen would improve the process control and possibly reduce energy consumption. Portable suspended solids meters would be beneficial in maintaining the desired MLSS (mixed liquor suspended solids) content in the aeration basins.
SLE&C Recommendation:
The Assessment Team endorses the COWI recommended project.
In addition, the laboratory required old equipment to be replaced and new equipment to be purchased in order to adequately monitor the operations and effluent discharges of the treatment plant. Discussions with laboratory personnel identified requirements for the following:
- New instrument for analysis of oil products;
- New instrument for analysis of surfactants (surface active substances);
- Portable instruments for dissolved oxygen, pH, nitrogen and phosphorus;
- Portable instruments for determining moisture content.
Laboratory personnel believe that operational efficiencies could be improved with the portable instruments; while the quality of laboratory data would be improved with the fixed instruments.
SLE&C is in agreement with the comments made by laboratory staff and recommends to include these additional instruments in the project.
WWT-19 Grit Chambers – Lines 2 and 3
Background:
Line 1 contains five (5) horizontal grit chambers which have sufficient hydraulic capacity and are reported to produce satisfactory grit removal. However, the chain and flight grit collection equipment is old and subject to frequent breakdowns.
Treatment Lines 2 and 3 each have four (4) aerated grit chambers. These aerated grit chambers have adequate hydraulic capacity, except for those of Line 2, which are slightly overloaded at maximum daily flows. The amount of grit removed from all aerated grit chambers, however, is insufficient – actual performance achieved is 84 – 88% vs. normal design requirement of 93-95%. Both aeration equipment and grit collection mechanisms are either under-designed or malfunctioning.
A relatively high grit content from Lines 2 and 3 passes downstream to succeeding treatment facilities. The grit settles out in the primary clarifiers, the aeration tanks and sludge digesters, which causes blockages and breakdowns.
Sand deposited in the digesters reduces the working volume which adversely impacts on digester performance – less volatile organics are stabilized and less gas is produced. At the present time there is no effective way of removing the sand from the digesters, except by manual rope and bucket. This causes extensive downtimes and loss of digester capacity.
COWI Recommendations for Improvement:
The project recommended comprises: complete reconstruction of the aerated grit chambers for Lines 2 and 3 and replacement of sand removal mechanisms, including aeration systems, scrapers, grit pumps, etc.
SLE&C Assessment:
During the site visit, structural condition of the grit chambers in Lines 2 and 3 was observed to be significantly better than the Line 1 equivalents, however, corrosion was noted.
During the limited time available for the inspection, the focus was on the most problematic facilities of Line 1; however, based on the data provided and what facilities could be observed in Lines 2 and 3, it is acknowledged that sand passing through the grit chambers is a significant problem. During the inspection, only Line 1 had primary clarifiers and aeration tanks out of service for maintenance. In these facilities, the build up of sand was observed, and plant staff reported that the problem is even more significant in Treatment Lines 2 and 3.
Efficient removal of grit is important to prevent unnecessary abrasion and wear of mechanical equipment and sand deposits in downstream processing units, especially in the anaerobic digesters where the performance of the digestion process is impaired. It is also critical to remove as much grit as possible for protection of sludge dewatering centrifuges and high-pressure progressive cavity and diaphragm pumps, which are easily damaged by grit.
The adverse effect of large grit deposits in the anaerobic digesters is of particular concern, since the plant is already operating with insufficient digester capacity. The effort required to rehabilitate the functioning digesters is staggering. In the history of the plant it was reported that only two of the tanks have been restored. One digester is currently being restored and has been out of operation for 18 months. Restoration requires that personnel must enter the tanks from a small hatch and manually shovel accumulated sand and trash into buckets to be lifted out through the hatch. The tank environment is hazardous to workers (oxygen deficient and a source of hazardous gases). The volume of the tank at 5,200 m3 allows the accumulation of extremely large quantities of sand and trash.
Reduction in sand passing through the grit chambers would reduce the quantity and improve the quality of primary sludge subsequently pumped to the digesters for stabilization.
SLE&C Recommendation:
The project recommended by COWI is endorsed by the SLE&C Assessment Team without exceptions.
WWT-1A Replacement of Screens - Line 1
Background:
Line 1 has five manually cleaned screens which have been in operation since 1965. They are completely deteriorated and need to be replaced. The bar spacing is 16 mm which means that only the very coarse material is screened out. Screenings are manually removed, taken to a comminutor and returned to the raw sewage inlet channel.
The unscreened material that passes through the coarse screen racks causes significant problems in downstream treatment units – primary clarifiers, aeration tanks, sludge digesters, and causes breakdowns of mechanical equipment.
Screenings are removed manually in a poor working environment in which plant staff is exposed to excessive hydrogen sulphide levels as well as to the risk of contracting disease.
Treatment Lines 2 and 3 contain eleven (11) coarse bar screens, 16 mm openings, which are mechanically cleaned. These screens also are deteriorated and need to be replaced. But Line 1 screens are most critical and need immediate action.
COWI Recommendations for Improvement:
Recommendations for the replacement of screens at Line 1 are:
- Replacement of existing coarse screens with two (2) mechanical fine screens;
- Installation of conveyors and compactor for screenings; and
- Air exhausting system and air treatment.
SLE&C Assessment:
During the site reconnaissance, if was confirmed that the existing screens for Treatment Line 1 are in poor condition. The rakes are in poor condition with significant corrosion observed and parts broken (raking tynes broken, screens bent).
Manual cleaning of the screens and the lack of trash handling systems (collectors and conveyors) results in inefficient, labour-intensive and hazardous (exposure to pathogens) working conditions.
The use of coarse bar screens with 16 mm openings is no longer sufficient for pretreatment of the wastewater for a large plant. Screening of wastewater protects downstream operations and equipment from damage. Further, due to the problems which can occur due to the recombining of comminuted materials, it is recommended that the coarse screenings be physically removed from the wastewater flow stream rather than re-introducing the shredded material into the sewage flow.
Mechanically cleaned screens with 6-8 mm openings are almost always specified for new plants of all sizes in North America. Mechanical cleaning, compared with manual cleaning, tends to reduce labour cost; improve flow conditions and screening capture; reduce nuisances; and, in combined sewage collection systems, better handle large quantities of stormwater debris and screenings.
Trash that is not captured by the screens adversely impacts the efficiencies of downstream processes. This was particularly apparent in the primary clarifiers where floating trash was observed on the surface of the wastewater, becoming entangled along the edges of the overflow. Debris was also observed wrapped around diffusers and other pieces of equipment in the aeration tanks that were under repair.
Screenings removed from Lines 2 and 3 are sent to the municipal incinerator for incineration, however, without facilities to remove water from the screenings, many complaints are received from the operators of the municipal incinerator.
Replacing the screens with modern equipment should improve the efficiencies of downstream operations and decrease the amount of maintenance and repair required for these operations.
SLE&C Recommendation:
The SLE&C Assessment Team endorses the recommendation of the COWI Report but suggests that further consideration be given to the number of screens to be provided.
COWI has recommended to install two mechanical fine screens, each with a capacity of 10,000 m3/h. It is noted that the five existing Line 1 coarse bar screens each have a capacity of 10,000 m3/h. The current average daily flow to Line 1 is 286,200 m3/d (11,925 m3/h) and the maximum daily flow is 441,700 m3/h (18,400 m3/h). It is recognized that future sewage flows are expected to decrease, but since this is still an unknown, it is suggested that considerations be given to the provision of three screens rather than just two, to meet maximum daily flows and to maintain treatment when one unit is down for maintenance and repair.
WWT-13 Sludge Digesters No. 1-4
Background:
Primary sludge from all three treatment lines is pumped to anaerobic Digesters No. 1 to 4 which are operated in the thermofilic temperature regime at about 55ºC.
The plant has a total of eight (8) digesters, Digesters No. 1-4 which were built in 1965 and Digesters No. 5-8 which where built after 1975. Only Digesters No. 1-4 are still in service; the other four were poorly built and are now completely deteriorated and have been taken out of service.
With only 4 digesters in service, there is insufficient digestion capacity to treat and completely stabilize all of the primary sludge from the plant. The primary digested sludge, therefore, still has a relatively large amount of solids, is only partially stabilized and is unhygienic.
In addition, the mixing systems of the four digesters (Digesters No. 1-4) is insufficient, which, together with the low solids retention time, results in low gas production of only 50-60% of normal production rates. The digester gas is used for heating the digesters and buildings, but it must be supplemented with 8,200 m3/d of natural gas to meet the total demand.
COWI Recommendations for Improvement:
Recommendations for the Digesters No. 1-4 upgrade project consist of:
- Repair of structures at the four digesters;
- Replacement of pipelines, pumps, gas pipelines and other equipment of sludge digesters;
- Installation of mechanical mixers, control equipment with gas-meters; and
- Installation of heat exchangers and modernization of the existing process of sludge heating.
SLE&C Technical Assessment:
At the site reconnaissance visit, anaerobic sludge digesters were inspected from the outside only (due to the nature of the vessels) and the following observations were made:
- Digesters No. 5 to 8 were in very poor condition; the tanks showed crumbling brick and concrete, degraded surface coatings and corroded metal fixtures and equipment;
- It was confirmed that Digesters No. 5 to 8 are not operational;
- The condition of Digesters No. 1 to 4 was found to be far superior despite their earlier construction;
- Plant personnel reported severe accumulation of sand and trash (non-captured screenings) in the digesters.
Our technical assessment of the four operating digesters shows that:
- The digesters are severely overloaded – actual loading is 1.86 kg VSS / m3d of digester working volume; the design loading rate for thermofilic digesters which are moderately mixed is about 0.80 kg VSS / m3.d; hence the SRT (solids retention time) or HRT (hydraulic retention time) is insufficient;
- In spite of the high overloading of the digesters, the VSS reduction achieved of 67% is relatively good; the typical range for thermofilic digestion is 45 to 72%;
- Gas production was reported as 0.243 m3 gas / kg VSS destroyed; normal gas production is in the order of 0.350 m3 gas / kg VSS destroyed at standard conditions (20oC, 1 atm.); in comparison, the actual gas production efficiency is about 69.4% of normal;
- COWI estimated that with the four refurbished digesters, natural gas consumption could be reduced by about 40-50%, or about 3,700 m3/d on average;
- Our assessment shows that the increased gas production by COWI may be overestimated; a more realistic increase of digester gas production may be 30 to 40%, which would reduce natural gas consumption by about 18.4 to 24.5% (due to its lower Btu, energy, value). The potential saving in natural gas consumption, therefore, could be about 1,760 m3/d.
In North America, most anaerobic digestion systems are designed to operate in the mesophilic temperature range (about 38oC); while in Europe, particularly with in-ground digester tanks, operation in the thermophilic range (about 55oC) is not uncommon.
Advantages claimed for thermophilic digestion include improved sludge dewaterability, increased pathogen destruction, and increased scum digestion. These claims have not been entirely substantiated. In North America thermophilic digestion has not been used very often because of:
- higher operating costs;
- lower process stability; and
- more stringent structural requirements (due to higher stresses associated with higher temperatures).
SLE&C Recommendation:
The anaerobic digestion process is a vital part of the operation of the wastewater treatment plant in order to:
- decrease the amount of sludge requiring disposal; and
- to inactivate pathogenic microorganisms and vectors to produce a product that is hygienic and amenable for application on agricultural land.
In view of the fact that the digesters are already in-place, and the structures are relatively sound (or at least in salvageable condition), the least cost solution in the short term is to refurbish the existing Digesters No. 1 to 4. We are therefore in agreement with the COWI Report recommendations in their entirety.
The long term objective should be to provide sufficient digester capacity to treat all co-mingled primary and secondary waste activated sludge produced at the plant. To this end, Plant personnel had indicated that:
- They had an alternate plan that involved replacing all the existing digesters with six new above-ground digesters (each 6,000 m3 capacity) to treat both primary and secondary sludge.
- This would allow all the existing digesters to be decommissioned as well as the aerobic sludge stabilization system.
No doubt, the construction of new, above-grade anaerobic digesters to treat all of the sludge from the plant may be a better option to refurbishing of existing digesters. The above-grade digesters would be operated in the mesophilic range of about 38oC for heat conservation and other reasons given above.
Given the age of existing Digesters No. 1-4 and the original construction flaws of Digesters No. 5-8, the construction of totally new digesters may be the preferred option. A detailed assessment of the two alternatives would be required with due consideration to 1) short term needs for maintaining the operation of the existing digesters, 2) space availability for new digesters, and 3) economics.
WWT-18 Sludge Dewatering (Digested Primary Sludge)
Background:
Anaerobic stabilized primary sludge from the digesters and aerobic stabilized biological sludge is pumped to the sludge lagoons some 9 to 15 km off site. Some of the digested primary sludge is mechanically dewatered by solid bowl scroll-type centrifuges; the dewatered sludge is transported by trucks to on-site sludge lagoons.
Sludge Lagoon Systems I and III are located about 9 km south-east of the wastewater treatment plant, and Sludge Lagoon System II is located some 15 km from the plant. Lagoon System III has been used for eight years after the Chernobyl catastrophe and is no longer in operation. The oldest lagoon system is System I. Both Lagoon Systems I and II are nearly full. The sludge lagoons are only emptied and restored gradually as the demand for additional storing capacity occurs and such sludge is disposed off on nearby agricultural land.
Annual sludge production after final consolidation in the lagoon to 40% D.S. will occupy 210,000 m3 of sludge lagoon volume, or 14 hectare at 1.5 m depth.
Reject water (supernatant) from the lagoons is pumped back to the head of the treatment plant. About 9,000 m3/d of sludge is pumped to the lagoons, while the reject water returned is about 3,000 m3/d. The reject water contains up to 1.5 g/L of suspended solids, which has a negative impact on the treatment plant performance.
There are no reserve sludge lagoons and the content of heavy metals in the sludge is too high to make it appropriate for disposal at agricultural land.
The sludge pipelines between the plant and the sludge lagoons are in bad condition and in need of immediate repair/replacement.
COWI Recommendations for Improvement:
The Report states that:
- mechanical dewatering equipment must be installed to reduce the amount of sludge that must be disposed off at the sludge lagoons;
- three (3) existing centrifuges (by Humbolt) have been installed, which are used to dewater anaerobically digested primary sludge; with all three units in operation, there is sufficient capacity to dewater all of the digested primary sludge;
- new sludge dewatering equipment for aerobically stabilized surplus activated sludge should be installed; specifically, three (3) centrifuges, or filter presses, with a capacity of about 130 m3/h each receiving 3-5% D.S., should be provided.
Comments by Plant Staff:
Plant personnel feel that dewatering of sludge solids is a key issue from the perspective of improving the performance of the treatment plant as a whole. Dewatering both primary sludge and surplus activated sludge will reduce the load on the sludge lagoons and substantially reduce the need for treating reject water from the lagoons.
Facility personnel feel that treatment of the reject water has a significant adverse impact on process efficiency and requires a large amount of increased energy. In addition, high costs are incurred for the operation of the pumping stations – pumping of sludge over long distances to the lagoons, and return of lagoon reject water back to the plant, monitoring, and maintenance and replacement of pumping equipment and the pipelines.
SLE&C Technical Assessment:
Currently, the reject water contains high levels of suspended solids, 1.5 g/L on average, because the lagoons are at capacity, preventing solids from settling out to produce a relatively clear supernatant.
The additional solids load on the plant, returned with the reject water from the sludge lagoons, amounts to about 108 tonnes/day which accounts for about 50% of the total biological sludge generated at the plant. This is a particularly heavy burden on the treatment capacity of the plant.
Daily sludge loadings to the sludge lagoons are given in Table 7.17 of the COWI Report as follows:
Dry solids, average – 231 tons SS/day
Sludge slurry volume, average – 8,910 m3/day
Sludge slurry concentration (calculated) – 2.57 % D.S.
The sludge slurry volume breaks down into approx. 7,530 m3/d of aerobically stabilized WAS and about 1,380 m3/d of anaerobically digested primary sludge. Some of the anaerobically digested primary sludge is dewatered by centrifuge; the sludge cake is trucked to the on-site sludge lagoons, while the centrate is returned to the aeration basins. Only one centrifuge was in operation at the time of the site visit by COWI staff in February 2000, while the other two units were down for minor repair work. Facility personnel, however, indicated that financing of the polymers is a major problem.
The impact of dewatering the additional 1,380 m3/d of anaerobically digested primary sludge is as follows:
volume of primary digested sludge – 1,380 m3/d
sludge slurry solids concentration – 2.57 % D.S.
density of sludge slurry– 1009.4 kg/ m3
specific gravity of D.S. – 1.45 (estimated)
amount of D.S. in sludge slurry – 35.8 t/d
centrifuge sludge cake solids concentration – 30% D.S.
sludge cake density – 1102.7 kg/ m3
typical solids capture in centrifuge – 95%
amount of D.S. in sludge cake, with 95% capture – 34.0 t/d
volume of water removed with sludge cake – 83 m3/d
volume of centrate (centrifuge overflow) – 1,274 m3/d
estimated solids concentration in centrate – 1.4 g/L
The above data serve to show that when all of the primary anaerobically digested sludge is dewatered:
1,380 m3/d of primary sludge slurry does not have to be pumped to the sludge lagoons;
about 108 m3/d of sludge cake containing about 34 t/d D.S. will be trucked to the on-site sludge lagoons;
about 1,274 m3/d of centrate containing about 1.4 g/L SS will be returned to the aeration basins.
The net impact on the sludge management process is:
- pumping costs for primary sludge slurry pumping to the sludge lagoons are eliminated;
- pumping costs for the portion of reject water from the consolidated primary sludge slurry from the lagoons to be returned to the plant would be eliminated;
- the amount of flow and solids returned to the aeration basins would be about 1,274 m3/d and the solids loading would be about 1,784 kg/d or about 1.8 t/d; this represents a significant decrease in suspended solids loadings on the plant over the existing solids loading contributed to the plant by the return of the lagoon reject water for primary settled sludge.
A similar analysis for dewatering aerobically stabilized waste activated sludge yields the following:
volume of waste activated stabilized sludge – 7,530 m3/d
sludge slurry solids concentration – 2.57 % D.S.
density of sludge slurry – 1,009.4 kg/ m3
specific gravity of D.S. – 1.45 (estimated)
amount of D.S. in sludge slurry, for 95% D.S. capture – 195.3 t/d
centrifuge sludge cake solids concentration – 10% D.S.
sludge cake density – 1,032.0 kg/ m3
typical solids capture in centrifuge – 95%
amount of D.S. in sludge cake, with 95% capture – 185.6 t/d
volume of sludge cake produced – 1,798 m3/d
volume of water removed with sludge cake – 1,670.2 m3/d
density of centrate slurry – 1004.1 kg/ m3 (S.G. D.S. = 1.45)
volume of centrate (centrifuge overflow) – 5,722 m3/d
amount of solids in centrate, at 95% capture – 9,767 kg/d
centrate solids concentration – 0.17%
estimated solids concentration in centrate – 1.7 g/L
The above data serve to show that when all of the secondary activated stabilized sludge is dewatered:
7,530 m3/d of stabilized secondary activated sludge slurry does not have to be pumped to the sludge lagoons;
about 1,798 m3/d of sludge cake containing about 185.6 t/d D.S. will be trucked to the on-site sludge cells, capacity permitting; or could be pumped to the sludge lagoons;
about 5,722 m3/d of centrate containing about 1.7 g/L SS will be returned to the aeration basins.
The net impact on the sludge management process is:
- power costs for pumping secondary sludge slurry to the sludge lagoons are eliminated or dramatically reduced;
- power costs for pumping the portion of reject water from the consolidated secondary sludge slurry from the lagoons to the plant would be eliminated or dramatically reduced;
- the amount of flow and solids returned to the aeration basins would be about 5,722 m3/d and the solids loading would be about 9,767 kg/d or about 9.8 t/d; this represents a significant decrease in suspended solids loadings to the plant over the existing solids loading contributed to the plant by the return of the lagoon reject water for secondary aerobically stabilized settled sludge (the existing SS loading as shown previously is 108 t/d);
- depending upon the ultimate disposal of the dewatered sludge cake, there could be an additional (minor) contribution to the plant in terms of slurry and solids loading from the reject water after final consolidation of the sludge solids has taken place.
Also, it is noted that the proposed dewatering systems require chemical coagulants for operations. These must be purchased in large quantities (COWI estimates 1.5 million USD/year: Ref 1, Annex 3.1 pg 35). As previously mentioned, KVK has had problems financing the purchase of coagulants for the sludge treatment processes in place. Whether purchased from Western manufacturers or those in the CIS, the costs are significant with respect to KVK’s financial position and problematic to meet on a regular basis.
SLE&C Recommendation:
As demonstrated in the above technical assessment of COWI’s recommendation to install two centrifuges for dewatering of aerobically stabilized secondary sludge, it is concluded that:
- power costs for pumping of sludge and return reject water will be reduced with the installation of the dewatering centrifuges; but additional power will be required for operation of the centrifuges;
- the solids loading on the treatment plant due to return of reject water from the lagoons would be significantly reduced from about 108 t/d to about 11.6 t/d, which is due to elimination of the return of reject water from the lagoons, but includes the return of centrate from the dewatering centrifuges;
- there would be a significant savings in power costs due to the elimination of sludge pumping and return pumping of the lagoon reject flows; however, new operational costs would be incurred as a result of handling and disposal of the centrifuge sludge cake, and the use of polymer (noted above) to condition the sludge for dewatering; the net impact will need to be determined during the project feasibility study;
- any savings in pipeline repair costs should not be counted on at this time, since they may need to be kept in service to handle all or a portion of the dewatered sludge;
- additional benefits may be achieved by dewatering a combined anaerobically digested primary sludge and the aerobically stabilized secondary sludge;
- pre-requisites for mechanical dewatering of digested sludge is to provide effective screening and degritting of the raw sewage, and to provide heated water at the centrifuge for periodic flushing and cleaning for the removal of grease.
In the long term, the future sludge management plan to be undertaken would certainly resolve the issue concerning the need for mechanical dewatering at the plant.
In the short term, based on the above assessment of the current situation, the SLE&C Assessment Team is in agreement with the COWI recommendation to install two, 130 m3/h, centrifuges (or plate filter presses) for the dewatering of aerobically stabilized waste activated sludge.
WWT-26B Sludge Disposal, Phase 1
Background:
About 9,000 m3/d of digested primary and stabilized secondary sludge is pumped to sludge lagoons for further consolidation and disposal. Clarified supernatant at a rate of 3,000 m3/d containing 1.5 g/L of SS (referred to as reject water in the COWI Report) is pumped back to the treatment plant for re-processing. Mechanically dewatered sludge (anaerobically digested primary sludge) is trucked to on-site sludge storage cells.
Sludge Lagoon Systems I and III are located about 9 km south-east of the wastewater treatment plant, and Sludge Lagoon System II is located some 15 km from the plant. Lagoon System III has been used for eight years after the Chernobyl catastrophe and is no longer in operation. The oldest lagoon system is System I. Both Lagoon Systems I and II are nearly full. The sludge lagoons are only emptied and restored gradually as the demand for additional storing capacity occurs and such sludge is disposed off on nearby agricultural land. However, the metal content (chromium, cadmium, nickel and mercury) of the sludge is relatively high and exceeds current regulations for land application of the sludge.
The oldest sludge lagoons, Lagoon System I, do not have any lining; Lagoon System II has a concrete lining and Lagoon System III has both concrete and plastic linings.
Annual sludge production after final consolidation in the lagoon to 40% D.S. will occupy 210,000 m3 of sludge lagoon volume, or 14 hectare at 1.5 m depth (ref. COWI).
COWI Recommendations for Improvement:
Recommendations comprise:
- reconstruction of the existing sludge lagoons (it is assumed this refers to Lagoon System I);
- rehabilitation of the drainage system (again, it is assumed this refers to Lagoon System I);
- design and construction of new sludge lagoons for disposal of dewatered sludge (this may imply providing additional sludge storage cells on the site of the treatment plant, or off site in close vicinity of the treatment plant).
Comments by Plant Staff:
As the lagoons are at capacity, reject water must be removed (decanted) in order to provide continued storage for new sludge.
The rate of sludge generation exceeds natural losses due to evaporation.
Because the sludge lagoons are at capacity their efficiency is poor and the reject water contains suspended solids in concentrations greater than desirable.
Rehabilitating the sludge lagoons by removing concentrated sludge and disposing of it (application on agricultural lands if heavy metal concentrations are not too high) would improve the settling efficiency of the lagoons and (may) improve the quality of any reject water.
Removal of sludge from the lagoons is a difficult operation to implement due to lack of resources for lagoon rehabilitation, transport of the sludge and identifying locations that can receive the sludge.
Additionally, the high metal concentrations evident in some of the sludge make their use on agricultural fields undesirable.
Long term storage of the sludge in permanent containment cells (i.e., landfills) is also not an option immediately available, (since it would require to be first dewatered to reduce the large volume currently being generated).
SLE&C Technical Assessment:
Due to time constraints and the remote locations of the sludge lagoons, they were not inspected as part of the site reconnaissance.
Disposal of wastewater treatment plant sludge to a lagoon system requires that lagoons are water retaining structures and that no leakage occurs to the groundwater below. Even for landfill applications, North American regulations require a liner and under-drain system to prevent flow of leachate into the subsoil and then into the groundwater. The permeability of the liner system must be 1x10-7 cm/s or less.
Lagoon System I is reported to have no liner, which is not desirable since it is a source of pollution to the groundwater in the area.
COWI reported a total solids load being discharged to the lagoons of 231 tonnes/day and a sludge slurry volume of 8,910 m3/d. On this basis, the annual production of sludge with a terminal solids density in the lagoon of 40% D.S., amounts to about 210,000 m3/year, as calculated by COWI. The additional lagoon capacity, therefore, for a five year operating period, is 1,000,000 m3.
SLE&C Recommendation:
COWI’s proposed project involving the reconstruction of the existing sludge lagoons, rehabilitation of the drainage system and design and construction of new sludge lagoons for disposal of dewatered sludge seems to be the most promising medium-term solution.
The SLE&C Assessment Team endorses the recommendations put forth by COWI.
WWT-21 Return Sludge Pumps – Line 1
Background:
Treatment Line 1 contains 6 return activated sludge (RAS) pumps. These pumps have been in operation since 1965 and are nearly completely deteriorated.
The pumps incur frequent breakdowns that are expensive to repair. But, even more importantly, when pumps break down, insufficient RAS is returned to the aeration basins resulting in unsatisfactory biological treatment.
The pumps are very inefficient and consume a lot of energy; motor capacities range from 55 to 75 kW.
COWI Recommendation for Improvement:
It was concluded that wastewater production in the next 10 years will decrease significantly (ref. Working Document No. 2 – Strategic Plan), and that only three (3) RAS pumps for Line 1 would be required to return sufficient RAS to the aeration basins for efficient biological treatment.
On this basis, COWI recommended to replace three (3) of the existing six (6) RAS pumps for Line 1 as follows:
- Installation of three (3) new RAS pumps directly in the lower sludge channel;
- Replacement of electrical equipment and valves in the lower sludge channel;
- Installation of instrumentation and control equipment and automation of the return sludge pumping systems.
SLE&C Assessment:
During the site inspection, it was observed that the six (6) RAS pumps in Treatment Line 1 are in very poor condition. It was concluded that complete replacement of the pumps and their auxiliary equipment would be the least cost solution to maintaining pumping capacity.
Facility personnel emphasized that this project is very important since the pumps and ancillary equipment are in critical condition, near the point of total failure, and the pump house itself appears to be structurally unsound.
of the facilities by our Assessment Team confirmed that the structural integrity of the building is question. The pump house, which is quite old, has been damaged by vibrations from the pumps, and due to the damp and corrosive environment, most of the concrete surfaces have deteriorated.
The pumps were found to be in poor condition; only two (2) units could be operated at the time of the visit, while the other four (4) were down for repairs.
From a process perspective, the RAS system is very important to achieve efficient biological treatment; the pumps return settled activated sludge from the secondary clarifiers back to the aeration basins. To maintain an efficient operation, the RAS must be returned before the microorganisms deplete the dissolved oxygen. Also, the RAS must be as concentrated as possible and the flow must be accurately measured and controlled.
SLE&C Recommendation:
Based on the observations of the Assessment Team, the recommendation of the COWI Report is hereby confirmed. In addition, SLE&C recommends that the pump house also be replaced.
WWT-4A Primary Clarifiers – Line 1
Treatment Line 1 contains 14 circular primary clarifiers, Line 2 contains 12 and Line 3 contains 12. All clarifiers are similar in design and construction, are 40 m in diameter and have an average water depth of 4.0 m.
The clarifiers in Line 1 have been in operation since 1965 and are almost fully deteriorated; only 50% or 7 out of the 14 units are still in working condition. Performance is inadequate – there is insufficient removal of suspended solids and floating material (scum and trash).
Mechanical installations, including the bridges, scrapers, drives, overflow weirs, grease removal systems, etc., are very corroded and need replacement. Discharge pipelines and discharge primary sludge pipes are broken and/or blocked with grit, trash and shredded screenings from the comminutor discharge.
COWI Recommendation for Improvement:
It was concluded that wastewater production in the next 10 years will decrease significantly (ref. Working Document No. 2 – Strategic Plan), and that only seven (7) primary clarifiers for Line 1 would be required to adequately pre-treat the anticipated reduced flows.
On this basis, COWI recommended to rehabilitate seven (7) of the existing primary clarifiers as follows:
- Repair of structures;
- Replacement of pipelines;
- Replacement of mechanical equipment, comprising access bridges, motors, scrapers, overflow weirs, grease removal systems, etc.; and
- Replacement of control panels and installation of modern control systems and monitoring equipment.
SLE&C Assessment:
The Assessment Team focused its inspection on Line 1 clarifiers with greater emphasis than on the other clarifiers in Lines 2 and 3 because of the old age of the Line 1 units.
Many of the primary clarifiers in Line 1 were out of operation and were dewatered, permitting visual observations of the concrete structures and equipment. The efficiency of solids removal of the in-service clarifiers could be inferred to some degree from the amount of residue and grit that remained in the out-of service clarifiers.
For the out-of-operation clarifiers, structural concrete was in poor to good condition with upper surfaces exposed to weathering showing the greatest deterioration. Metal structures and equipment were observed to be corroded. Sand and trash were observed having accumulated in sheltered areas in the clarifiers, on equipment and metal structures.
For the units in operation, accumulation of trash on the surface of the water was observed, in some cases covering up to 25% of the surface area. Trash was often caught on the lip of the overflow weir. Solid particles were observed overflowing the clarifier to a degree that would not be considered optimal.
Using the data presented in the COWI Report, our technical assessment leads to the following observations:
- At current flow rates, the average unit clarifier loading rate is 32.5 m3/m2.d, which is at the low end of the normal design rate of 32 to 41 m3/m2.d; at maximum daily flow, the max. unit clarifier loading rate is 50.2 m3/ m2.d and the peak hour rate is 55.4 m3/ m2.d, which compares with the normal maxi. design loading rate of 61 m3/ m2.d; hence, on the basis of hydraulic loading, seven clarifiers are enough to handle existing wastewater flows; with a reduction in future flows, it may be possible to operate successfully with six units, while one clarifier is out of operation for maintenance and repair;
- The average water depth of 4.0 m is consistent with the design depth of 3.6 m (range 3-5 m);
COWI reported performance data as follows:
- SS (suspended solids removal) – 27.6%
- BOD5 (5-day biochemical oxygen demand) – 21.3%
- COD (chemical oxygen demand) – 14.8%
A 1989 WPCF(1) survey of primary sedimentation tank performance found that for a unit overflow rate of 32.5 m3/ m2.d the range in SS removal was 35 to 73% and the average was about 60%; in comparison to the performance of SS removal at the Bortnicheskaya WWTP, it can be concluded that the primary clarifiers are not performing well and higher removal rates should be possible.
(1) WPCF denotes the Water Pollution Control Federation of the United States, now known as WEF – Water Environment Federation.
Any increase in SS removal in the primary clarifiers would also improve the removal efficiencies for BOD5 and COD associated with volatile suspended particles.
Of a more general note, it should be observed that primary sedimentation treatment offers low cost suspended solids and BOD5 removal, especially in cases where the raw sewage contains a high proportion of settleable solids, as is typical with sewage containing significant food processing, or similar wastes.
SLE&C Recommendation:
Based on the observations of the Assessment Team and the subsequent technical analysis of the primary clarifiers of Line 1, it is confirmed that SLE&C concurs with the COWI recommendation to rehabilitate seven of the existing fourteen primary clarifiers.
WWT-11A Secondary Clarifiers – Line 1
Background:
Treatment Line 1 contains 14 circular secondary clarifiers, Line 2 contains 12 and Line 3 contains 14. All clarifiers are similar in design and construction, are 40 m in diameter and have an average water depth of 4.0 m.
The clarifiers in Line 1 have been in operation since 1965 and are very which affects the performance of the overall biological treatment process for Line 1.
Mechanical installations, including the bridges, scrapers, drives, overflow weirs, grease removal systems, etc., are very corroded and need replacement. Discharge pipelines and discharge secondary sludge pipes are partly broken and/or blocked.
The secondary clarifiers for Line 1 are in urgent need of rehabilitation and require installation of new equipment.
COWI Recommendation for Improvement:
It was concluded that wastewater production in the next 10 years will decrease significantly (ref. Working Document No. 2 – Strategic Plan), and that only seven (7) secondary clarifiers for Line 1 would be required to adequately settle activated sludge solids from the effluent of the aeration tanks.
On this basis, COWI recommended to rehabilitate seven (7) of the existing secondary clarifiers as follows:
- Repair of structures;
- Replacement of pipelines;
- Replacement of mechanical equipment, comprising access bridges, motors, scrapers, overflow weirs, grease removal systems, etc.; and
- Replacement of control panels and installation of modern control systems and monitoring equipment.
SLE&C Assessment:
The Assessment Team focused its inspection on Line 1 clarifiers with greater emphasis than on the other clarifiers in Lines 2 and 3 because of the old age of the Line 1 units.
The condition of the secondary clarifiers in Line 1 was observed by the Assessment Team to be similar to the primary clarifiers.
Surface trash and overflow of suspended solids were observed to be significantly less than for primary clarifiers, as would be expected at the final stage of a treatment process.
Using the data presented in the COWI Report, our technical assessment leads to the following observations:
- At current flow rates for Line 1, the average unit clarifier loading rate is 16.3 m3/ m2.d, which is at the low end of the normal design rate of 20 to 34 m3/ m2.d; at maximum daily flow, the max. unit clarifier loading rate is 28.8 m3/ m2.d, which compares with the normal maximum design loading rate of 40 to 60 m3/ m2.d; hence, on the basis of hydraulic loading, the fourteen secondary clarifiers (all on line) are well within the lower end of the normal design criteria;
- The solids surface loading rate is given as 4.2 kgSS/ m2.h, which is at the low end of the maximum solids loading rate range normally used by design engineers – 4 to 6 kgSS/ m2.h; rates up to 10 kgSS/ m2.h or more have been observed in some well-operated plants;
- the above data show that the secondary clarifiers in Line 1 are lightly loaded and have adequate capacity for existing loadings.
Based on the foregoing, it can be concluded that sufficient secondary clarifier capacity in Line 1 is available, but as noted in the COWI Report, improvements are required to the hydraulic functioning of the clarifiers and, especially, the proper operation of the mechanical equipment.
Secondary clarification is a very important unit operation in the performance of the biological activated sludge process. Clarifier performance is critical for meeting TSS (total suspended solids) BOD5, and occasionally TKN (Total Kjeldahl Nitrogen) discharge criteria.
SLE&C Recommendation:
In view of the projection for reduced flows over the next 10 years (ref. Working Document No. 2 – Strategic Plan), and the possibility to close down at least a part of the Line 1 facilities, the Assessment Team is in agreement with the COWI recommendation to rehabilitate only seven of the existing fourteen clarifiers.
It is noted, however, that the foregoing analysis applies to fourteen secondary clarifiers in operation. Hence, the final decision as to the number of secondary clarifiers that should be rehabilitated should depend upon an assessment of current needs.
WWT-20 Sludge Thickeners
Background:
Surplus activated sludge (WAS) from the secondary clarifiers is thickened by gravity thickeners before being treated by aerobic stabilization.
Line 1 thickeners were built in 1965, are severely deteriorated and are no longer used. Line 2 includes five (5) thickeners, each 40 m diameter similar to secondary clarifiers. Line 3 thickeners are still under development and have not been used.
All WAS is thickened in the five Line 1 gravity thickeners.
COWI Recommendations for Improvement:
The COWI feasibility study recommended that “The sludge thickeners should be replaced either by new ‘traditional’ sludge thickeners or by mechanical sludge thickeners, …”. The recommendation is to replace all existing gravity thickeners by mechanical sludge thickeners.
SLE&C Technical Assessment:
Only five (5) thickeners are used to thicken surplus activated sludge from the entire plant.
The thickener overflow rate (total sludge volume generated) is 0.16 m/h [m3/ m2.h], which is at the low end of the overflow rates typically used for secondary (waste activated) sludge – the usual range is 0.167 m/h to 0.333 m/h.
Therefore, based on hydraulic loading criterion, the 5 thickeners have sufficient hydraulic capacity for gravity settling of secondary sludge.
The critical design parameter, however, for gravity thickening is the floor loading in terms of weight of total solids per unit area per unit time.
Typical solids loading rates are 20 kg/ m2.d to 30 kg/ m2.d [4 – 6 lb/ft.2.d].
The actual solids loading rate at the plant, for the entire sludge volume generated, is 50.3 kgSS/ m2.d, which by far, exceeds the normal loading rates of 20 to 30 kgSS/ m2.d.
Hence, the existing 5 thickeners are severely overloaded based on solids loading criterion.
The underflow solids concentration achieved is 2.5% D.S., which is on the low side; but experience shows that, in general, the typical range in solids underflow concentrations is 2 to 3% D.S.
The above analysis shows that gravity thickening of waste activated sludge is inefficient and generally not used for this application. It is more frequently used for primary sludge, and a mixture of primary and waste activated sludge.
The existing five thickeners have very large diameters, and long hydraulic retention times, that can lead to gas formation, sludge flotation, generation of odours, particularly when the wastewater temperature exceeds 20oC.
The SLE&C Assessment Team, therefore, concurs with the COWI Report recommendation to replace (or initially, partially replace) existing sludge thickeners with “mechanical” sludge thickeners.
The COWI Report does not state what type of mechanical thickeners are to be provided; only that two (2) new thickeners are to be provided. It is further suggested that these new mechanical thickeners might be used for both primary sludge and surplus activated sludge.
SLE&C Recommendation:
The Assessment Team endorses the COWI recommended project. However, since there is some uncertainty concerning the type of equipment to be provided, its size and capacity, and also its specific application, it is recommended that a pre-design (definition phase) study be included in the project. North American practice is to use DAF (dissolved air flotation) for thickening of waste activated sludge, and more recently, by centrifugation in a solid bowl conveyor/scroll-type centrifuge, or by gravity belt filter press.
Both thickening processes, but more so the centrifuge belt filter press, require the use of polymers to enhance the performance of the thickening process. It is recognized that, similar to the proposed sludge dewatering project, the need for polymers in the process will be a significant burden to KVK. Given the financial situation of the organization, purchasing the required polymers may be difficult.
The short term objective, therefore, would be:
- to treat primary sludge and secondary sludge separately as per current practice consisting of:
- treatment of primary sludge directly by anaerobic digestion;
- possibly thickening primary sludge before digestion using existing gravity thickeners, if practical;
- treatment of secondary waste activated sludge by mechanical thickening, DAF or centrifuge, followed by aerobic stabilization.
In the long term, the objective could be:
- to treat primary sludge by gravity thickening using existing gravity thickeners, if practical, followed by anaerobic digestion;
- to treat secondary waste activated sludge by mechanical thickening, DAF or centrifuge, followed by aerobic stabilization;
- ultimately, to teat waste activated sludge by mechanical thickening, followed by combined treatment of primary sludge and thickened waste activated sludge in anaerobic digesters;
- dewatering of anaerobically digested sludge by centrifuge, or belt fiter press, and disposing of the solids cake to sludge lagoons and/or a landfill; the centrate woud be returned to the aeration tanks.
WWT-7A Aeration Tanks – Line 1
Background:
Line 1 contains six (6) horizontal aeration tanks with each tank subdivided into four (4) channels; channel dimensions are 12 m Wx120 m Lx5 mD. These tanks consist of in-ground concrete basins which have been in operation since 1965. It was reported by COWI that structurally the tanks are in poor condition and that several aeration tanks are partly demolished.
Diffused aeration is used for the supply of oxygen and mixing of the aeration tanks. The air is distributed through a linear arrangement of ceramic diffusers which, because of their age, are assumed to consist of coarse bubble diffusers as opposed to the more recent fine bubble diffusers. It was reported by COWI that existing diffusers are inefficient in distributing the air throughout the aeration tank. This means that many of the diffuser plates may have serious fouling problems.
COWI also reported that the very large motors (1,350 kW) for the air blowers do not have frequency regulation, which means that the air supply cannot be regulated to meet the demand, hence power consumption is inefficient.
There is no control of the air supply to the tanks and no monitoring equipment (either on-line or portable) available to allow the operator to immediately assess the status of the oxygen levels in the tanks. In addition, it was reported that the air pipes and valves for regulation of the air supply to the different channels within a tank are severely deteriorated (corrosion).
Wastewater production is expected to be significantly reduced over the next 10 years, and it is expected that at least part of the facilities of the Line 1 treatment train can be shut down (ref. Working Document No. 2 - Strategic Plan). As a result, it was proposed to only rehabilitate three (3) of the existing six (6) aeration tanks in Line 1.
COWI Recommendation:
The project recommended by COWI comprises:
- Replacement of three (3) of the existing seven (7) blowers by blowers with adjustable air flow;
- Replacement of automatic valves for air control and of airways; three (3) units;
- Installation of the monitoring and control equipment, i.e., oxygen meters (6 units), pressure meters, valve position indication, etc.;
- Replacement of filter plates (coarse bubble ceramic diffusers) at three (3) aeration tanks by new fine bubble diffusers;
- Replacement of pipes and valves; and
- Reconstruction of structures.
SLE&C Assessment:
The site inspection by our Assessment Team confirmed the generally poor condition of the aeration tanks in Treatment Line 1.
There is widespread corrosion of metal hand-railings, air distribution piping and appurtenances. Concrete walkways generally are in poor condition. The structural concrete