Proposal by Russia to delete hot sub-spot Hot sub-spot name South-West Wastewater Treatment Plant

Transcription

1 Proposal by Russia to delete hot sub-spot 18.4 LAND 14/2009, Document 6/3/Rev.1 ATTACHMENT 1. Hot sub-spot name South-West Wastewater Treatment Plant 2. Location Block 2, 123, Volkhonskoye shosse, St. Petersburg 3. Reasons for hot spot inclusion Phosphorus concentrations in the WWTP effluent are higher than required by 4. Reason for hot spot deletion 4.1. Technical description of the South-West Wastewater Treatment Plant (SWTP) The South-West WWTP was put into operation on 22 September 2005; its design capacity is 330,000 m 3 /day which is sufficient to treat the wastewater collected from the area inhabited by over 700,000 people. The plant was built to comply with treatment quality recommendations. The design flow rate is 16,300 m 3 /h, the maximum flow rate 32,600 m 3 /h which is twice the design capacity of the plant and is intended for primary clarification and biological treatment of wastewater. The pumps, screens and grit channels are designed for 45,900 m 3 /h. The surplus water is pumped to expansion tanks. Maximum flow rate can be maintained for not more than two consecutive to ensure proper treatment quality. WWTP equipment The main pumping station has 11 pump units (total capacity being 45,900 m 3 /h and there are 4 mechanical 100 mm bar screens to protect the pumps from big-sized inclusions and clogging). Wastewater is pre-treated at 8.8 mm bar screens. The screenings are pressed and disposed off at a landfill. Then wastewater passes through 8 sand traps, the total volume of which being 3,180 m 3. There it is treated for 12 minutes at the design flow rate or 4 minutes at the maximum flow rate. The design wastewater speed in the sand trap is 0.3 m/sec. Air feed is controlled to comply with water amount in the rate from 40 to 100%. Sludge is removed by pumps (one per each chamber) installed on two mobile platforms. The sand traps have two flushing units. Floated matter and grease are removed with scrapers. The scrapers, pumps and flushing units are turned on and off automatically within pre-set time intervals. Mechanical treatment After pre-treatment, wastewater is treated at 4 primary radial-flow sedimentation tanks (diam. 54 m, depth 5.3 m). The retention time is 3 hours at design flow rate. Sludge from the primary sedimentation tanks can be pumped to the sludge treatment room and back to the distribution chamber to enhance the formation of VFA (volatile fatty acids) required for biological absorption of phosphorus. Stream recirculation increases the retention time in the primary sedimentation tanks and the concentrations of incoming suspended solids in the tanks up to g/l. The maximum capacity of mechanical treatment facilities is 45,900 m 3 /h, while that of the primary sedimentation tanks and biological treatment facilities is 32,600 m 3 /hour. 13,300 m 3 /h of surplus wastewater resulting from maximum wastewater flow rate during rains and snow 1

2 LAND 14/2009, Document 6/3/Rev.1 melting is stored in the tanks (radial-flow sedimentation tanks, total volume 48,550 m 3 ) equipped with mixers to prevent precipitation. When the inflow decreases, wastewater accumulated in the tank is pumped to the primary clarifiers and then to biological treatment. Overflow of wastewater from the storage tanks is designed for the periods of continuous rains or melting of snow. When the expansion tanks are emptied, sludge and water are pumped to the distribution chamber or to the biological treatment facilities. The biological treatment of wastewater takes place in 6 aeration tanks divided into anaerobic, anoxic, transitional anoxic-aerobic and aerobic zones to ensure biological removal of phosphorus, nitrogen and organic pollutants. The retention time in the aeration tank is 6.8 hours at the design capacity of 16,300 m 3 /h: 1 hour in anaerobic zone, 1.5 hours in anoxic zone, 1.2 hours in anoxic-aerobic zone and 3.1 hours in aerobic zone. Mixed sludge from the aeration tanks enters 8 secondary radial-flow sedimentation tanks where activated sludge precipitates; the design retention time is 6.7 hours. The sludge scrapers in the sedimentation tanks are designed for continuous operation. The treated effluent is disinfected in the UV exposure unit located before the outlet chamber. Sludge treatment. Grease and scum collected in the sand traps are pumped to the gravity grease separation unit from which separated grease comes to a 20 m 3 storage tank (retention time being 48 hours at normal wastewater flow rate) and incinerated together with the sludge thereafter. Excess activated sludge is pumped to four 18m diam. sludge thickeners, the design retention time of activated sludge in the thickeners is 18 hours. The design dry matter content in thickened sludge is 2.5 %. Primary sludge and thickened excess activated sludge go to 2 tanks with mixers (the volume of each tank is 500 m 3 ) for mixing. The design retention time is 10 hours or less depending on the sludge amount. The tanks have an aeration system to be used in the event of odour, the flow being 1.4 m 3 per 1 m 3 of the tank volume. Mixed sludge is dewatered at 4 centrifuges using polymer flocculants (6 kg/t). The design separation efficiency in terms of solid particles is 96% if dry matter content in dewatered sludge is max.75%. Separated water goes to the distribution chamber prior to the primary sedimentation tanks. There are 2 tanks, both having 540 m 3 capacity, that are used to store dewatered sludge during 48 hours breaks in operation. In 2007 a sludge incineration plant was put into operation in the SWTP area. Ash can be either dumped without any risk of pollution or used for the production of construction materials. The calorific value of sludge and grease is estimated at MJ/kg of organic DS. Advanced effective cleaning of flue gases is supplemented by steam and power generation capacity Actions taken to meet recommendations, including the implementation of the best available technologies and best environmental practice. In addition to the design wastewater treatment process, the following measures were implemented one by one in at the SWTP: raw sludge fermentation in primary sedimentation tanks (December 2005), chemical phosphorus precipitation in primary sedimentation tanks (March 2007), and chemical phosphorus precipitation in sludge mixing tanks to eliminate the secondary contamination caused by separated water (March 2006). Raw sludge digestion. Acid digestion means that the primary sludge is retained in anaerobic conditions during 4-8 and recirculated. This time is enough to digest complex disperse materials into volatile fatty acids (VFA), mainly acetic and propionic acids, but not enough for VFA to transform to methane and CO 2. 2

3 LAND 14/2009, Document 6/3/Rev.1 To ensure efficient operation of SWTP primary sedimentation tanks as digestors, the following parameters are maintained continuously: sludge age 6 + 1, sludge layer thickness approximately m, redox 300 mv, concentration of suspended solids at the outlet of primary sedimentation tanks below 100 mg/l, raw sludge recirculation ratio 5% of the influent wastewater. Chemical phosphorus precipitation in sludge mixing tanks. Biological removal of phosphorus is achieved by cultivating special biocenoses in the activated sludge and creating rather tough conditions for the biological treatment process. It is possible to remove phosphorus efficiently by extracting excess sludge containing polyphosphates and alternating anaerobic and aerobic conditions that fosters great accumulation of phosphorus in the cell bodies. Under stress conditions (alternation of anaerobic and aerobic zones) activated sludge bacteria can remove phosphates in large amounts, up to 10% by DS. The phosphates extracted with biomass are removed from the system together with excess activated sludge. Phosphates release in the mixing tanks upon long contact of thickened excess activated sludge and raw sludge; their concentrations can reach mg/l. Agent supplementation to the sludge mixing tanks was introduced at the SWTP in March The purpose was to remove secondary phosphate pollutants in separated water after dewatering at centrifuges. Without chemical supplementation the secondary pollution accounted for 15 to 20% of the total phosphates content in wastewater fed to the aeration tanks (РО 4 -Р concentration in treated water reached 300 mg/l). Addition of iron sulphate solution to the sludge mixing tanks helped reduce the phosphates concentration in separated water to 50 mg/l. the average dose of Ferix-3 in 2008 was 1.9 kg per 1m 3 of sludge. Chemical phosphorus precipitation in primary sedimentation tanks. Fe 2 (SO 4 ) 3 coagulant of the Finnish company Kemira-Eco was selected upon a series of laboratory tests for chemical precipitation of phosphorus, and since March 2007 it has been fed regularly into the primary sedimentation tank s distribution channel. The optimal chemical dose is 45 g per 1m 3 of wastewater. Chemical precipitation allows only partial of phosphates concentration at the outlet of primary sedimentation tanks. Most of phosphates are removed from wastewater in the aerobic zone of aeration tanks. Sufficient content of dissolved oxygen in the first aerobic zone is a key factor for maximum absorption of phosphates. Different observations and tests at the SWTP have demonstrated that the optimal content of dissolved oxygen in the first aerobic zone is mg/l Capital investments SWTP construction cost is 181 million Euro. Now the pilot full-scale tests of chemical phosphorus precipitation at SWTP are completed. Based on the test results, a stationary chemical dosing unit is being implemented. The project is carried out in the framework of bilateral cooperation between Russia and Finland (contract No.1009/07D dated ). Joint financing: EUR 476,222 by John Nurminen Foundation; EUR 243,352 by SUE "Vodokanal of St. Petersburg" Discharge of pollutants in the previous 4 years Monitoring results Mean concentrations at the SWTP outlet in 2008 were: BOD5 4.2 mg/l, total nitrogen 7.4 mg/l, total phosphorus 0.4 mg/l. Total phosphorus concentrations at SWTP inlet and outlet in are shown in the diagram below. 3

4 LAND 14/2009, Document 6/3/Rev.1 Total phosphorus concentrations at SWTP inlet and outlet in Concentrations, mg/l 6 5,7 5,1 5 4,6 4, ,8 2 2, , Years 0,4 5. Assessment of plant operation compliance with recommendations 28E/5 Wastewater treatment (over 100,000 inhabitants) ) Year BOD 5 N tot. Р tot. requirements requirements requirements**) ,2 97, mg/l mg/l 2,8 44,0 0.5 mg/l ,4 98 or 80% or 70-80% 2,1 50,0 or 90% ,1 98 7,8 73 0, ,7 98,3 7,4 75 0, months of ,3 98 8,8 76 0,2 96 ) compliance to be achieved by 31 December ) Recommendation 19/2: standard of 1.5 mg/l for phosphorus concentration in the treated effluent 6. Pollutant discharge monitoring and control programmes Discharge of treated effluent is monitored in compliance with the document: Procedure and schedule of official control over the operation of treatment plants, effluent quality and the influence of effluent on hydro-chemical regime of water bodies. 4

5 LAND 14/2009, Document 6/3/Rev.1 Procedure and schedule of SWTP operation control Sampling point Plant inlet Plant outlet Effluent discharge point Neva Bay in the Gulf of Finland, sea area in the radius of 500m Sampling method Automatic sampler Manual sampler Sample type Daily average Grab sample Grab sample Periodicity of sampling Daily, every 2 hours Once in 10 Once in 30 Once in 10 Once a month (August- October) Monitored parameters рн, SS, ash content of SS; P-PO4; N-NH4; N- NO3; N-NO2; COD Ntot. Ptot.; dry residue; ash content of dry residue; Cu; Fe; Ni; Zn; Hg; Al; Mn; synthetic surfactants; phenols; chlorides Total BOD. Temperature; oil; dissolved oxygen; total coliforms; thermo-tolerant coliforms; coliphages; helminth eggs, pathogenic microflora Temperature; ph; odour; colour; COD; BOD5; SS; dissolved oxygen; Ntot.; Ptot.; oil; total coliforms; thermo-tolerant coliforms; helminth eggs; coliphages; pathogenic microflora; Mn; Fe; Hg. 5