Technical Assessment of Proposed Pollution Mitigation Measures

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8. Technical Assessment of Proposed Pollution Mitigation Measures

 

In this section, the proposed mitigation measures to be carried out at the Krasnoflotskaya Wastewater Treatment Plant, referred to herein as the MWTP, are assessed technically from the following perspectives:

 

- Confirmation of need based on observations made during the site reconnaissance;

- Confirmation of need based on data provided;

- Verification that the proposed approach is appropriate. 

 

In general, the Assessment Team observed that the individual processes are not working well and unit operations are in poor condition due to age and wear. As most of the facilities were in operation during the site reconnaissance, some of the indicators of their condition (corrosion and structural failure) as well as indicators of the overall system performance (accumulated grit, sludge and trash in downstream processes, fouling of equipment, etc.) could not be observed. For those units which were out of service for maintenance and repair, the exposed equipment was observed to be in poor condition. One might expect that the in-service units are similarly in poor condition.

 

A number of observations that affect many of the projects were observed by the Assessment Team. These include the following:

 

- Overall plant operations are extremely labour-intensive with personnel working long hours in poor conditions. The screens (bar racks) and primary clarifiers are constantly being attended to by staff raking trash off the screens and sweeping floating debris and scum off the clarifiers.

- Accurate flow measurements of the treatment plant influent may not be obtained since one side of the channel steel insert plate for the Parshall flume is out of position which distorts the dimension at the throat of the flume. The accuracy of the flow measuring device is important as it affects the operation of the activated sludge plant. 

- The sludge dewatering operations (centrifuge) has a polymer feed system with issues affecting its operation. 

- Most metal equipment and machinery is made from steal, not stainless or corrosion-resistant steel. This decreases the life expectancy of the equipment and machinery; although, replacement and even repair of the equipment may not be possible because the Vodokanal lacks sufficient funds to undertake the work.

- Instrumentation and control systems for automatic operation of various parts of the plant including on-line monitoring instruments of various process parameters are non-existent at the MWTP. Depending upon the size of the wastewater treatment plant, a certain amount of instrumentation and control is desirable to improve the operation and performance of the plant. Automatic control of the dissolved oxygen in the aeration tanks, for example, is possible and  may result in energy and money savings. 

 

The projects discussed below relate directly to those introduced in Section 7. Only prioritized projects are discussed in detail. 

 

Because only the facilities of the MWTP were surveyed during the reconnaissance phase of the operation, comments will be restricted to the eight projects proposed for this facility. 

 

Main Wastewater Treatment Plant:  Intallation of Screens

 

Background:

 

As previously described in Section 4, the MWTP contains two treatment lines, Line 1 consisting of pre-treatment facilities only and receiving about 20% of the total flow, and Line 2 which includes all unit operations of a complete activated sludge plant. The pre-treated flow from Line 1 is discharged to the inlet flow of the aeration basins of Line 2 for further treatment.

 

Line 1 facilities were commissioned in 1982, while Line 2 facilities were commissioned in 1987. Both lines include screening facilities followed by grit chambers and primary clarifiers.

 

Line 1 screening facilities consist of three (3) coarse bar screens (racks) with 16 mm clear spacings between bars; Line 2 has four (4) comminuting bar screens (inclined barminutors), also with 16 mm clear spacings between bars.

 

Screenings from Line 1 are removed manually by rake and small trash containers. Those from Line 2 are shredded and re-introduced into the wastewater flow.

 

Recommendation by Gorvodokanal Management:

 

Screening facilities for each treatment line are to be replaced. It is recommended to construct a new head house for each line and to install mechanically cleaned coarse bar screens with bar spacings of 6 to 8 mm. More details of the project are currently not available.  Another scenario includes the integration of Lines 1 and 2 including sharing a single screening facility.

 

SLE&C Assessment:

 

During the site reconnaissance, if was confirmed that the existing screens are in poor condition. Metal components are severely corroded and various parts were observed to be broken.

 

Manual cleaning of the screens in Line 1 and the lack of mechanical trash handling facilities (collectors and conveyors) results in inefficient, labour-intensive and hazardous (exposure to pathogens) working conditions.

 

The manually collected screenings from Line 1 are transferred to an open box truck trailer without dewatering and are periodically transported to the local municipal landfill for disposal.

 

Coarse screens with 6 to 8 mm bar spacings should be provided for the protection of plant equipment against blockage or physical damage. Because existing screens allow so much trash to pass through with the wastewater, this 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 and requiring staff to constantly remove this debris.

 

Also, due to problems which can occur in downstream processes, recombining comminuted (shredded) material with the wastewater should no longer be practised. 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 coarse screens, opposed to manual cleaning, are preferred and tend to reduce labour costs, improve flow conditions and screening capture, reduce nuisances, and reduce possible flooding and overflows due to clogging.

 

SLE&C Recommendation:

 

The SLE&C Assessment Team endorses this project. Replacing existing screens with modern equipment should improve the efficiencies of downstream operations and decrease the need for maintenance and repair of equipment related to these operations.  The provision of automatic trash removal systems is strongly recommended.

 

Main Wastewater Treatment Plant:  Aeration Tanks

 

Background:

 

Secondary treatment at the Smolensk Vodokanal main sewage treatment plant consists of:

 

- two (2) aeration tanks, each containing four (4) 200m long channels; total channel length is 800 m;

- three (3) secondary clarifiers (circular, sedimentation tanks);

- blower building with four (4) centrifugal air blowers (superchargers).

The National Experts indicated that aeration tanks are operated in the "contact stabilization" mode of the activated sludge process. In this mode:

- the first channel, 25% of total volume, is used for regeneration of the return activated sludge (RAS) from the secondary clarifiers;

- the next two channels, 50% of total volume, are used for aeration of dispersed supply of wastewater; and

- the last channel, 25% of total volume, is used for aeration of the total mixture of wastewater and activated sludge (commonly known as the mixed liquor).

 

Air is introduced to the aeration basins by diffused aerators located uniformly along the length of each channel.

 

Plant staff had visually inspected aeration piping and determined that they produce small and well-dispersed air bubbles along the whole length of the pipes.

 

Four (4) air blowers are operated in the on/off mode to supply the right amount of air to the air diffuser piping. The value of excess oxygen is analyzed in the laboratory.

 

Recommendation by Gorvodokanal Management:

 

It is recommended to modernize the aeration tanks and to partition aeration channels to allow for nitrification / denitrification to occur and to improve the removal of phosphorus compounds.

 

SLE&C Assessment:

 

The site reconnaissance confirmed the generally poor structural condition of the aeration tanks. Metal equipment and parts are corroded and concrete structures are in poor condition.

 

Air blowers are old and consume a lot of electricity. Blowers are controlled manually from local control stations without air flow measuring equipment.  Also, there are no on-line dissolved oxygen monitoring probes and samples must be collected manually and analyzed in the laboratory.

 

A project to repair the existing diffusers in Section A (activated sludge process channel) of the aeration tanks was undertaken last year.  Over 700 air diffuser pipes were replaced and the balance treated with hydrochloric acid to remove build-up of scale and blockages.  The acid treatment failed because most of the blockages were caused by sand, which was resistant to this type of treatment.  This project did not achieve long-term success and replacement of the diffuser pipes with new equipment was subsequently undertaken with successful results.

 

Our technical review of the aeration basins revealed the following:

 

- Plant personnel indicated that the plant is operated in the "contact stabilization" mode of the activated sludge process. This does not appear to be accurate and will need to be confirmed by the National Experts or the Technical Director of the MWTP.  The contact stabilization process has two phases; in the first phase, the contact tank, settled sewage is mixed with the reaerated return from the stabilization tank  and aerated for about 30 to 90 minutes (based on ADWF, average dry weather flow); in the second phase, the stabilization tank, settled sludge from the secondary clarifiers is returned and aerated for 3 to 6 hours (based on ADWF) in the sludge aeration tank. During this time the absorbed organics are utilized for energy and production of new cells.

- As it is currently stated, the two centre channels of each aeration tank only serve to aerate settled raw sewage.  This represents unused capacity since little to no reduction in organics is expected to occur here.  In view of the high dissolved oxygen content, long retention time (4.3 hours on total flow without activated sludge recycle), and potential for nitrifying bacteria to become established, it is possible that most of the nitrification achieved in the process occurs here.

- The contact stabilization process has generally been used for smaller plants treating domestic sewage.  Application of the process to a mixture of municipal and industrial wastes is not always successful; process efficiency drops when the BOD component is largely soluble.

- The existing aeration tanks represent the conventional activated sludge process, and can be readily converted to the "plug flow" process or one of its modifications consisting of the "tapered aeration" or "step aeration" activated sludge processes.  Under existing conditions, the plug flow process is the simplest to implement.  For an average daily flow of 134,000 m3/d and 50% recycle of settled activated sludge, the hydraulic retention time in the 2 aeration tanks is 6 hours which is well positioned between the range of 4 to 8 hours (based on ADWF) typically used for the plug flow activated sludge process.

- With the plug flow process, air supply can be controlled to optimize the process.  Sufficient dissolved oxygen levels should be maintained in the final two channels of each aeration tank to achieve a high degree of nitrification.

- In implementing the nitrification process it is important to check the buffering capacity of the wastewater since about 7.14 mg/L of alkalinity as CaCO₃ is destroyed per mg/L of NH₄⁺-N oxidized. The residual alkalinity should be sufficient to maintain the pH above 7.2.

- Nitrogen in the effluent has an adverse impact on the receiving stream. Organic nitrogen and ammonia nitrogen present in raw sewage should be nitrified in the existing activated sludge process.  An ammonia concentration as low as 2.5 mg/L is toxic to fish and any concentration in the effluent is considered critical.  Process modifications are therefore necessary to achieve nitrification.  Ammonia in the treated effluent will also affect the chlorination process and significantly increase the required chlorine dosage; stoichiometrially, 7.6 parts of chlorine are required to oxidize 1 part of ammonia nitrogen in aqueous solution.

- The conversion of each mole of ammonia (18 g of NH₄ per mole) produces 1 mole of nitrate (62 g of NO₃ per mole); in terms of nitrogen, each gram of ammonia nitrogen will produce one gram of nitrate nitrogen. Nitrate is not desirable in the final effluent since it will stimulate the growth of algae and other aquatic plant life.  A concentration of 0.3 mg/L of NO₃ in the receiving waters is considered critical to the biostimulation process in surface waters.  Eutrophication is of most concern in lakes rather than in a river since nutrients do not tend to accumulate in a flowing river other than in sediment or slack waters.

- Since algal growth requires both nitrogen and phosphorus, the questions remain as to  which element will limit growth and whether it is essential to remove both nutrients.  It is not possible to remove nitrate nitrogen (denitrification) in the conventional activated sludge process.  Following nitrification, which may occur in the activated sludge process, or in a separate nitrification step, the nitrates are anaerobically converted to nitrogen gas (denitrification) in a separate activated sludge process. Typically, methanol is added as the carbon source to allow the energy reaction to go forward.

- Denitrification in a suspended growth reactor using methanol as the carbon source, following combined carbon oxidation-nitrification treatment, is a common denitrification process. The anoxic mixed denitrification reactor is typically followed by an aerated stabilization tank with an hydraulic retention time of about 50 minutes (based on ADWF). This tank is sometimes omitted but serves to remove any residual carbon due to excess dosing of methanol. The flow is then passed through an aerated nitrogen stripping channel with a design retention time of about 5 minutes (based on ADWF), and then discharged to the denitrification clarifier for solids separation. Settled denitrified sludge is returned to the front of the anoxic mixed denitrification reactor; while some of the sludge is wasted to maintain the mass balance of solids within the process.

- Several alternative denitrification systems include the attached growth systems (upflow, deep media, anoxic filter; Rotating Biological Contactor) and the combined carbon oxidation-nitrification-denitrification systems using wastewater or endogenous carbon sources.

- Because of the price of methanol, there has been much interest in utilizing the combined carbon oxidation, nitrification and denitrification process in a single sludge system with no intervening clarification steps. The organics present in the domestic wastewater or endogenous respiration of the biological sludge serve as the alternate carbon source required for denitrification. Various flow sheets have been investigated and applied full-scale, including but not limited to:

 

- a system utilizing endogenous respiration in a sequential carbon oxidation-nitrification-denitrification system with sludge recycle from the secondary clarifier;

- the same system as above but with an aerobic cell of up to 30 minutes hydraulic retention time (based on ADWF) placed before the secondary clarifier for the purpose of nitrogen stripping and to improve settling;

- systems using wastewater carbon in alternating aerobic/anoxic modes;

- aerobic/anoxic sequences in oxidation ditches;

- denitrification in an alternating contact process (tested and applied in Denmark);

- the Bardenpho Process developed in South Africa for nitrogen removal using both wastewater and endogenous carbon for denitrification; and

- alternating aerobic/anoxic systems without internal recycle.

 

- Total nitrogen removals of 90 percent have been achieved with several of the above-noted systems with wastewater temperatures above 20^oC. The system best suited for application at the Smolensk Gorvodokanal Wastewater Treatment Plant remains the subject of an in-depth feasibility study of how best to utilize the unit operations available at the plant.

- Excess phosphates (above the amounts required for organism growth) can be readily precipitated in the activated sludge process by the addition of either alum (aluminum sulphate) or iron compounds (ferrous sulphate, ferric sulphate, ferric chloride, and pickle liquor).  Reactions are pH dependent for the various metal salts; for ferrous iron, the optimum pH is about 8 and good removal has been obtained between pH 7 and 8.  Residual phosphates after the aeration tanks can be removed in the secondary clarifiers.

 

If the aeration tanks were rehabilitated, the benefits would be significant.  Improving the overall treatment performance and achieving nearly complete nitrification, and possibly partial denitrification, would be particularly beneficial.  A reduction in energy use would also be desirable.

 

SLE&C Recommendation:

 

SLE&C is in agreement with the recommendation of the corporation to rehabilitate the aeration tanks and to modernize the aeration equipment.

 

It is further recommended that a process study be carried out to establish the best way to utilize the existing facilities for the purpose of organic carbon removal, oxidation of ammonia nitrogen through nitrification and reduction of phosphates by chemical precipitation.

 

To remove nitrate nitrogen, it is our opinion a second-stage activated sludge plant is required. This would entail a costly expansion of the existing plant, followed by significantly higher annual operating costs. A study, however, should be initiated to determine the feasibility of  utilizing the existing aeration basin tankage to achieve denitrification. This could perhaps be based on a system where the wastewater carbon is used in an alternating aerobic/anoxic mode, or, perhaps, the tankage could be adapted to the Bardenpho process which utilizes both wastewater and endogenous carbon for denitrification.

 

At this stage, SLE&C does not recommend to implement a second-stage activated sludge plant for denitrification be constructed at the Smolensk main wastewater treatment plant due to the costs involved, but does endorse a feasibility study to be undertaken to determine if the existing aeration tankage can be adapted to a denitrification process without the need for additional major capital works.

 

The possibility of implementing denitrification in existing tankage was proposed by facility personnel.  It is our opinion that implementing such measures are at best partially successful and cause significant problems with recycle sludge. 

 

Main Wastewater Treatment Plant:  Sludge Dewatering

 

Background:

 

The information available on sludge handling and disposal is incomplete. Based on data received from the National Experts, it is inferred that the MWTP includes the following facilities:

 

1. screenings and grit from Line 1 and Line 2 facilities are manually collected and disposed of at the municipal landfill;

2. two (2) sediment compactors (gravity thickening tanks);

3. three (3) hydrocyclones;

4. two (2) anaerobic digesters (out of service);

5. six (6) solid bowl centrifuges;

6. three (3) aerobic mineralizers for fugacity (either aerobic stabilization tanks or sludge conditioning tanks);

7. fifteen (15) sludge ponds.

 

It is uncertain how sludge produced in the treatment process is handled. The document from the National Expert states that crude sediment (primary settled sludge) and excess activated sludge (from secondary clarifiers) are treated together before ultimate disposal in the on-site sludge lagoons.  Final dewatering of combined primary and waste activated sludge occurs in the lagoons.  Clarified supernatant is returned to the front of the aeration tanks while settled out solids (organic and inorganic) remain as a deposit in the lagoons.

 

Recommendation by Gorvodokanal Management:

 

To completely replace the existing centrifuge dewatering facilities with new filter presses and associated polymer feed systems.

 

SLE&C Assessment:

 

The handling, treatment and disposal of the sludges generated at the wastewater treatment plant is probably the most important aspect of wastewater treatment.  If the sludge cannot be removed at the rate at which it is generated in the process, be it the primary settled sludge from the pre-treatment clarifiers, or waste activated sludge from secondary clarifiers, then the plant will not function at that treatment rate.  Odours will be generated and the effluent quality will not be met.

 

It has been noted that at this plant primary settled sludge and excess activated sludge are being treated together; the sequence of operations, however, is unclear.  Usually sludge treatment and disposal options would consist of:

 

- thickening of primary and secondary sludges, either by gravity thickeners, air flotation, or centrifuges;

stabilization by anaerobic digestion, aerobic digestion (limited to waste activated sludge and small plants), sludge lagoons, or lime treatment;

- dewatering by drying beds, centrifuge, vacuum filter, belt filter, filter press, and others;

- disposal to impoundment reservoirs (lagoons), landfill, dedicated land as soil conditioner.

 

Anaerobic digestion is no longer used because the two digesters are in disrepair and no longer operational.

The existing dewatering centrifuges, while extensive, are partially out-of-operation and expensive to maintain.  There are frequent breakdowns and insufficient (or no) polymer is added to condition the feed.  The success of sludge dewatering by centrifuge, or any of the other mechanical dewatering operations mentioned above, is highly dependent upon conditioning of the sludge feed.  By conditioning the sludge feed with a polymer, a dryer sludge cake can be produced and the solids capture efficiency of the process will increase.

 

At the site reconnaissance, it was observed that existing sludge lagoons are being filled up and additional sludge storage space is required.

Dewatering both primary sludge and surplus activated sludge will reduce the load on the sludge lagoons and increase the capacity for handling emergency situation. A new and more efficient dewatering process is therefore urgently required. It would also eliminate the existing high maintenance costs and most likely reduce energy consumption in comparison to an equal amount of treatment.

 

Mechanical dewatering methods currently in use for the wastewater treatment industry are:

 

- belt press filtration;

- centrifugation;

- vacuum filtration; and

- pressure filtration (filter plate press); and

- other less well known methods, such as the screw press.

 

Pressure filtration is a good alternative for processing sludges with poor dewatering characteristics. Some advantages and disadvantages of pressure filtration are:

Advantages:

 

- highest cake solids concentration, with associated lowest sludge hauling and disposal costs;

- little operator attention during dewatering phase of cycle;

- cake solids concentration is relatively independent of feed sludge solids concentration;

 

Disadvantages:

 

- batch operation;

- high capital costs;

- large area required with special foundation design;

- in some cases, filter cloth preparation and washing may be operator intensive.

 

To further reduce the solids content requiring disposal and minimize production of  odours from the sludge lagoons, it is also important to stabilize the sludge by reducing its organic content before dewatering.  Sludge stabilization is generally achieved by digestion. Anaerobic digestion is the most commonly used system for the digestion of primary and mixtures of primary and waste activated sludges.  Aerobic digestion has been used for stabilization of waste activated sludge at small plants.

SLE&C Recommendation:

 

The SLE&C Assessment Team concurs with the corporation's recommendation to install new sludge dewatering facilities and associated polymer feed systems. Whether filter presses are the right choice must be confirmed in the pre-engineering study.

 

It is recognized that mechanical dewatering systems require chemical flocculants for efficient operations which must be purchased in relatively large quantities. As previously mentioned, Gorvodokanal has had problems with the financing and purchase of polymers, whether from Western manufacturers or those in the CIS.  However, the need for chemicals is common to all mechanical sludge dewatering systems, and is an operational cost which must be accounted for in the annual operating budget.

 

Main Wastewater Treatment Plant:  After-purification of Wastewater with Filters

 

Background:

 

The project proposed by the utility denotes the installation of two sand filters for the removal of additional suspended solids from the final effluent. Since the wastewater has already received primary and secondary treatment, the additional filtration step is commonly referred to as tertiary treatment.

 

No information is available on treated water effluent characteristics.

 

Tertiary filtration of the final effluent is sometimes necessary to meet the sustainable receiving water assimilation capacity.  This, however, is not a problem at Smolensk were there is a relatively high low flow dilution factor.

 

The purpose of the project was to reduce elevated levels of suspended solids in the discharge, thereby also reducing the BOD.

 

Recommendation by Gorvodokanal Management:

 

To construct two (2) granular media sand filters, associated by-pass channels and associated works (building).

 

SLE&C Assessment:

 

There is insufficient information available upon which to make a sound judgment.  Only a conceptual plan and a rough estimate have been prepared for this project to date.  Although the type and size of the sand filters has not been defined, the estimate is based on similar projects implemented elsewhere.

 

It is acknowledged that tertiary filtration will improve the quality of the final effluent.  The question to be asked is whether this is the best and most economical way to achieve it at the MWTP of Smolensk.

 

Based on our assessment of the activated sludge plant aeration tanks and the recommended project for sludge dewatering, it is concluded that:

 

- efforts should be made and money spent on improving the existing activated sludge plant, i.e., conversion of the contact stabilization process to a plug flow or step feed activated sludge plant;

- improving and controlling the rate of activated sludge return to the aeration tanks;

- upgrading the diffused air piping in the aeration tanks and controlling the air supply to maintain the required dissolved oxygen levels throughout the aeration tank channels;

- installing measuring devices where needed for proper process control;

- sludge handling and treatment facilities be upgraded so that any short-falls here do not impact on treatment plant capacity;

- since the BOD loading by industry (17,000 kg/d) represented 50% of the total BOD load entering the wastewater treatment plant in 2001, it would be of significant benefit to plant operations to have industry characterize its wastewater discharges so that utility staff can assess where pre-treatment or sewer surcharges are warranted or more economical than upgrading the plant itself.

 

SLE&C Recommendation:

 

Based on the findings of the SLE&C Assessment Team summarized above, it is the team's recommendation that this project not proceed at this time.

 

Main Wastewater Treatment Plant:  Ultraviolet Light Disinfection

 

Background:

 

The MWTP contains chlorination facilities, including a chlorine contact basins with two (2) channels.

 

Plant personnel have not provided sufficient reasons for the discontinuation of the disinfection process. It is suspected that chlorine dosages were quite high to reach breakpoint chlorination because of the relatively high residual ammonia nitrogen content. Also, the benefits of UV-disinfection over the use of chlorine gas may have been a factor in the decision. Some of the more relevant benefits are:

 

- it will eliminate the formation of organochlorine and trihalomethane compounds which are toxic to fish and other biotic species;

- there will be no chloramine residual in the treated wastewater effluent, which is an active chlorine compound and toxic the aquatic life;

- the overall quality of the final effluent will be improved which has an immediate benefit on the water quality of the receiving river;

- the dangers posed by the use, storage and handling of a very toxic chlorine gas will no longer exist.

 

Facilities for dechlorination are not currently available.  This process had never been used to eliminate the chlorine residual in the chlorinated effluent.

 

Recommendation by Gorvodokanal Management:

 

To install an Ultra Violet Light (UV Light) disinfection process, which would replace the existing chlorination process.

 

SLE&C Assessment:

 

With the chlorination process suspended, because of the harmful effect of residual chlorine (chloramines) on aquatic organisms, pathogens are present in the discharges to the river.

 

The pathogens would be eliminated by the installation of an Ultraviolet (UV) light disinfection system. Given the level of pathogens in the treated effluent, and the impact of such on downstream users, the treated wastewater effluent should be disinfected.

 

Disinfection using chlorine followed by dechlorination is one method to achieve both a low concentration of pathogens and low chlorine residual in the effluent.  It does not, unfortunately, eliminate the production of organochlorine compounds. 

 

The alternative to chlorination/dechlorination is to switch to the use of UV light for disinfection.  Such a facility, which is quite complex, would be desirable although a full feasibility study and assessment of costs are necessary. 

 

SLE&C Recommendation:

 

Assuming full nitrification can be achieved with recommended modifications to the activated sludge plant process, SLE&C recommends to re-instate the chlorination process and possibly to de-chlorinate the effluent after the chlorine contact tank until such time as a feasibility study of the UV light alternative can be produced and fully evaluated.

 

Main Wastewater Treatment Plant:  Sludge Disposal

 

Background:

 

The Smolensk wastewater treatment plant has fifteen (15) sludge ponds.  These are long and narrow ponds which receive a combined mixture of primary and secondary sludges following partial stabilization and dewatering to reduce the daily sludge volume requiring  disposal.

 

The ponds are fairly deep and the water level can be controlled by an effluent weir in a weir outlet chamber. The ponds serve to provide further biological treatment to oxidize organics and to settle out suspended solids. the clarified liquid at the surface is decanted on a continuous basis and returned to the aeration tanks in the plant. Settled solids remain and over time compact in the ponds.

 

The ponds also serve as emergency storage basins to hold raw sewage in case of problems in the plant and the need for short-duration by-passes.

 

The plant has insufficient sludge pond storage capacity to meet future needs and has already implemented the use of two temporary sludge storage areas (channels).

 

It is unclear whether the plant currently also uses sludge drying beds to consolidate sludge solids.

 

Recommendation by Gorvodokanal Management:

 

Expansion of existing sludge storage ponds and construction of additional sludge drying beds.

 

SLE&C Assessment:

 

Final sludge ponds serve as an integral part of the wastewater treatment plant.  Combined primary and waste activated sludge following pretreatment, is discharged to the sludge ponds either by gravity or by pumping and the supernatant (clear liquid from the surface of the ponds) is returned to the front of the aeration tanks for further treatment. The settled solids generally are retained in the ponds; hence the sludge ponds are normally referred to as sludge thickening lagoons.

 

Generation of odours and contamination of the groundwater regime are two principal disadvantages of sludge thickening lagoons. But without sufficient capacity or an alternate sludge disposal plan, the entire capacity of the wastewater treatment plant is in question.

 

If there is insufficient land area available at the treatment plant site, filled up sludge ponds could be restored by removing solids, either by pumping or by dredge, for disposal on farmland, if feasible, or as landfill elsewhere.

 

SLE&C Recommendation:

 

Although the information available is limited, the SLE&C Assessment Team is in agreement with the corporation's recommendation to install additional sludge ponds, as well as additional sludge drying beds.

 

The previously prepared study should be revisited and serve as the basis of a feasibility study.

 

Main Wastewater Treatment Plant:  Outfall Reconstruction

 

Background:

 

At present, the treated effluent from the MWTP is discharged to the Dnieper River via a river bank, above-surface, concrete outfall.  Previously, an in-stream outfall in the centre of the river was the location of the effluent discharge; however, this outfall was damaged and has not been replaced.

 

Recommendation by Gorvodokanal Management:

 

Restoration of the underwater pipeline for discharge of the effluent to the centre of the river.

 

SLE&C Assessment:

 

The objective for discharge of the effluent is to incur the least adverse impact on the river. Since the river is capable of absorbing some pollution by natural self-purification and sedimentation, the mixing zone in the river should be kept to a minimum. This can best be achieved by a diffused outfall located in the deepest section of the river where the river flow velocities are also high.

 

An on-shore, at the river bank above the water surface, outfall is the least desirable.  It has many disadvantages, a primary one being the channeling that results along the shore.  The effluent does not mix well with the river flow, hence the mixing zone is very elongated and can extend for long distances downstream of the outfall.

 

SLE&C Recommendation:

 

The project to reconstruct the submerged river outfall and to incorporate a diffuser at the center of the river is endorsed by the SLE&C Assessment Team.

 

It is an important project that will have benefits to the quality of water in the immediate vicinity downstream of the plant.  A diffused river outfall should mitigate current adverse effects on aquatic life at the river bank.