Project IV. Public Utility Gomelvodokanal

10.5. Project IV.  Public Utility Gomelvodokanal

10.5.1. Project Description

As of January 1, 2001, the population of the City of Gomel was 478.5 thousand people, including population of 445.0 thousand people (93%) comprising the residential sector, which is connected to the sewage system.  Industrial and municipal effluents are treated at the municipal treatment facilities.

Gomel municipal wastewater treatment facilities which first became operation in 1968, provides complete biological treatment and has a daily productivity rate of 90 thousand m³/day. In 1980, the productive capacity of the wastewater treatment facilities was increased up to 180 thousand m³/day.

The facility uses conventional processes and technologies, including mechanical, biological (aeration activated sludge tanks) treatment and decontamination of wastewater (there is post-treatment or tertiary treatment of sewage water).  The primary sludge and excess activated sludge is processed through a sludge reservoir, compactors, methane-tanks and sludge drying beds (along with ditch water storage ponds); the sludge digesters which were part of the original design of the project were never constructed.  That is why until recently, the seventh section of the aeration tanks has been used as an aerobic stabilizer to provide some sludge treatment.  Currently the raw sediment (from the primary clarifiers) is pumped directly to the sludge drying beds and the reject or decanted water is pumped to the “front” of the wastewater treatment facilities.  The equipment is primitive, obsolete (for installations of such productive capacity), inadequate for processing sediments in emergency situations and practically non-serviceable.  Decant or reject water from the sand and sludge drying beds is also pumped to the “front” of facilities, thereby increasing the hydraulic load on the wastewater treatment plant.  The wastewater treatment plant is in satisfactory and up state only due to a high level of exploitation, servicing and qualified process inspection.

The mechanical treatment complex includes screen facilities with mechanical rakes, grit chambers, pre-aerators and primary clarifiers.  There are five mechanical rakes in the rake section. Four horizontal two-cell grit chambers are used to remove sand and other mineral substances.  With the help of hydraulic elevators, the sand is removed to the sand grounds and then after drainage, to the sludge drying beds.  There are four primary radial clarifier tanks which are 40 meters in diameter and 4.0 meters of depth.  The aeration tanks are the main unit used in biological treatment.  The seven sections of aeration tanks are constantly supplied with air from the compressed air plant.  There are seven air blowers each with a capacity of 360 m³/min.  The aeration tanks (Sections 1, 3, 4, 5, 6, 7) are equipped with a disk pneumoaerators; the second section uses filter cartridges.  The biological treatment complex includes five final settling tanks, each 40 meters in diameter and two sludge compactors, each 24 meters in diameter.

The aw sediment and excess sludge are accumulated on the sludge drying beds (the ponds have a total area of 13.4 hectares whereas 42 hectares are needed for current conditions).  The operational service efficiency of the mechanical treatment process reaches 40-50% and the biological treatment efficiency is 90-100%. Disinfection of the treated wastewater is carried out in contact reservoirs which are serviced by four chlorinators.  After disinfection, the treated effluents are discharged to a branch duct (the Milchanska ditch) which is piped to the Uza River which flows to the Sozh River.  The total length of the line up to the Sozh River is 6000 meters.

In 2001, industrial effluents discharged to the municipal wastewater treatment facilities comprised about 20% (9561.9 thousand m³ of industrial effluents compared with 45117.4 thousand m³/year of residential sewage).

Table 10.10 :Volumes of Gomel Municipal Effluent Discharges (m³ per month) Processed by Effluent Treatment Plant during the Period from July 2001 to June 2002

Discharge of treated wastewater takes place continuously.  Monitoring of the discharge is carried out two times a month.  Point sampling and average daily sampling are taken every day in order to conduct the analysis.  All points of discharge are under supervision.

The data summarized below gives wastewater quality, operating practices and efficiency of functioning of wastewater treatment facilities.

Table 10.11 :Indices of Efficiency for Wastewater Treatment at Wastewater Treatment Facilities of the Town of Gomel in 2001

Table 10.12 : Qualitative Characteristics of Natural Water and Effluent at Control Points of Discharge from Wastewater Treatment Facilities of Gomel for 2001

The following conclusions are based on an analysis of the data and the results of the investigation:

The wastewater treatment facilities are hydraulically overloaded as stated above; they are also overloaded with regard to COD, phosphates, ammonia nitrogen, oil products, fats and other pollutants.  The toxicity of the activated sludge, in the incoming industrial effluents, exceeds 1.5-2.5 times the admissible limits.  The high level of pollutants inhibits the biological treatment process and, as a result the sludge index has been increased to160-175, thereby exceeding the criterion which is90-120.  It should be noted that the sludge index is reduced between the point of entry and the outlet of aeration tank. However the qualitative characteristics of the incoming wastewater is acceptable for biological treatment with respect to pH, BOD_20, COD and the content of biogenic elements.

The BOD₂₀ and suspended solids of the wastewater are below the specified limits.  This is evidence of considerable dilution by industrial effluents with low concentrations of contaminants (in terms of the indices examined) as well as stormwater runoff and underground waters, coming into sewerage system this is confirmed by the high specific volume of wastewater – 420-450 l/person/day.  This is one of the reasons of hydraulic overload of wastewater treatment facilities.

The environmental data for the wastewater treatment facilities show that the grit chambers and primary clarifiers have loadings below their hydraulic carrying capacity and the volume of the seven-section aeration tanks ensures sufficient aeration time for complete biological treatment both with regeneration (operation condition) and without regeneration.  The operating practices for the functioning aeration tanks are characterized by medium dose of sludge – 3.0-3.5 g/l (aeration tank – 1.5-2.0; regenerator – 4.0-4.5) with recirculation 0.3-0.5 (optimally 0.45).  At the same time, the volume required for the regenerator comprises 0.35-0.4 of the aeration tank volume.  Sludge loading is 198-212 mgBOD₂₀/g per day, in other words it is within the medium (optimal) load; the age of the sludge is 6.5-7 days.

The wastewater treatment facilities function at the upper limit of their technological capabilities and provide near design efficiency of wastewater treatment for suspended solids – 93% and BOD₂₀ -  93-95%.  Reconstruction of the aeration system and sectionalization of the aeration tanks have contributed considerably to the quality of the operation.

The following upgrades provide for maximum and rational use of the technological capacity of wastewater treatment facilities, intensification and optimization of the operating process subject to minimum construction of new facilities and complete utilization of capacity with construction of the third phase:

1.       Immediate construction of 2-3 sheets of sludge drying beds (drainable).

2.      Simultaneous construction of a stabilizer for primary sludge and activated sludge, which will be also    used after the complete reconstruction of the sludge complex.

Implementation of these facilities (listed in points 1 and 2 above) will produce a slight reduction in the load on the wastewater treatment process by eliminating the need to treat excess sludge.

3.        Installation of a sludge dehydration process for sediments using state-of-the-art filter-presses, which   will ensure maximum reliability for its functioning.

4.    Development of a program to reduce the effluent volume and pollution loads from enterprises   (especially with regard to effluents treated to standard quality), and the tightening of the covers of the drain shafts and sewage header covers located within the area of influence of underground water.

5.      Currently, it is possible to use aeration tanks without regenerators, since the BOD₂₀ in the incoming wastewater does not exceed 150 mg/l.

6.      On completion of the construction of the mechanical sediment dehydration system, retain some of the drainage sludge drying beds as emergency back-up and demolish others to allow the area to be available for future developments of the existing wastewater treatment facilities.

10.5.2. Mitigation Measures

Reduction in the pollution level of effluent, discharged to the Dnipro Basin, require realization of the measures, presented in Table 10.13.

Table 10.13 : Environmental/Mitigation Measures for Gomel Vodokanal for 15 years

10.5.3. Feasibility Analysis

The feasibility analysis of the water-protective measures was carried out by comparing the economic results of these measures with the costs required for their realization.  For Gomel Vodokanal, the following input data was used:

- Investment required for reconstruction and expansion of the treatment facilities – $US16380 thousand.

- Operational (current) costs – $US 200 thousand.

The respective input data are presented in Annexes 1-3 and used in the calculations in Table 10.14.

Calculation of mono-pollutant (М):

М = ∑ D_i V_i = 31620 c. t.

Calculated per year М₁ = 6324 c. t.

Economic estimate of the ecological damage averted (D_a):

D_a = 275*1.75*6324 = $US 3043 thousand

Calculation of increment of lost financial benefit (I)

I is assumed in professional judgment as 10% of D_a

I = $US 304 thousand

Calculation of the economic result of the water-protective measures (E_r):

E_r = D_a + I = 3043 + 304 = $US 3347 thousand

Calculation of the reduced costs (∆C)

∆C = Е * К + Т_c = [0.12 * 16380] + 200 = $US 2166 thousand

Calculation of the net annualized economic effect (∆U):

∆U = E_r - DC = 3347 – 2166 = $US 1181 thousand

The payback period (Т) for the investment into water-protective measures is:

Т = К / ∆U = 16380/1181 = 13.8 years

Based on environmental design practices, the threshold of effective return for investments into wastewater treatment facilities is 14 years.  Therefore, the suggested project with the estimated payback period of 13.8 years is economically effective and justified.

Table 10.14 : Reduction in Total Mass of Mono-Pollutant to be Achieved in Pollution with Effluent from Municipal Treatment Facilities within Five Years

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