Water treatment using fast clarification plants with application of weighting agents, Melnikovo settlement
Reconstruction of the water treatment system in Melnikovo settlement, Priozersk area, Leningrad region using fast clarification plants and applying weighting agents. The reconstruction increased the efficiency of water treatment and allowed the optimisation of the maintenance expenses.
Solution to which problem
Melnikovo settlement (population: 1746 people with a development plan for 10% expantion). Water in Melnikovo settlement is also used by the community facilities and enterprises: outpatient clinic, kindergarten, school, groceries, pharmacy, boiler house.
Water supply is provided by water supply networks with water columns for separate houses and central water supply networks for apartment (up to five storeys) building, social facilities and enterprises.
The source for the water supply is Vuoksa river. River water is characterized by increased turbidity and color, it contains a significant amount of water-soluble organic substances. A characteristic feature of water chemical composition is a presence of iron and manganese in concentrations close to maximum permissible concentration (MPC).
Physical and chemical parameters of the water quality in Vuoksa river
The water intake site is located in the southern part of Melnikovo settlement on Vuoksa riverside. Water treatment facilities were commissioned in 1965.
Water treatment facilities before reconstruction:
- First stage pump station with pump equipment
- Process building including:
- Second stage pump station
- Chlorinator house
- Vertical settling tanks
- High-rate filters with sand-and-gravel bed
- Underground storage tanks for clean water
Water treatment station scheme was designed as two-staged. Before reconstruction, the vertical settling tanks have already been out of operation, coagulation was not performed. Mechanical filtration through sand-and-gravel filling was applied. Water sterilization using sodium hypochlorite solution was performed before water supply to high-rate filters.
Water intake and water treatment facilities of Melnikovo settlement were under operation over fifty years without repair, and by 2013, the majority of the structure's facilities had already exceeded their safe usage date.
The source for the water supply is Vuoksa river. Upon the results of observations performed by North-West Weather Control and Environmental Monitoring Service FSBI, Vodokanal production association, Vuoksa river water along the whole length (from the border with the Vyborg region to the entry) is characterized as “moderately polluted” and rated as 3rd class (Water pollution index (WPI) varies from 1.33 to 2.1–2.25). River water has a higher level of permanganate oxidizability, turbidity, manganese and iron. In a number of components the water does not correspond to the requirements for drinking and fishery water sources.
Water treatment facilities did not provide the source water treatment as per regulatory requirements of SanPiN 126.96.36.1994-01.
The company selected the usage of two-staged treatment technology with settling tank and application of weighting agents to increase settling velocity, implemented as part of “AllianceEco-500” complex.
The technology is developed according to the requirements of SanPiN 188.8.131.524-01 DRINKING WATER and assumes cleaning against mechanical impurities, reagents injection and their mixing with raw water, clarification (accelerated sedimentation of floccules using microsand), filtering using pressure filters and sterilization.
On the described facilities, the water after coagulation enters the fast mixing chamber, to which microsand is supplied. It is used as a core during flocculation, provides large surface area and improves flocculation process, and also acts as a ballast. Sand-weighted floccules have unique settlement parameters (vertical speed is up to 40–80 m/h compared to 10 m/h for standard solutions). After adding microsand and passing the maturation champer, the flow enters a thin-layer settling tank zone, from which the settled grit (microsand and sediment) is removed for separation at hydraulic cyclone. Sand, separated from sediment in hydraulic cyclone, returns to the system for reuse.
The main process module of “AllianceEco” complex is a fast clarification plant (FCP) – efficient high-rate clarifier, in which microsand is used as a core during flocculation. Microsand provides large surface area and improves flocculation process, and also acts as a ballast. As a result, sand-weighted floccules have unique settling parameters that allow to design clarifiers with the high speed upward flow and short contact time.
Fast clarification technology has many advantages:
- High treatment efficiency: reduction of turbidity by over 90%;
- Compact size compared to other technologies;
- Small amounts of general construction works;
- Adaptability: quick reaction to the change of the source water quality;
- Very short startup time: less than 10 minutes;
- Reduced need for maintenance, easy access to each unit;
- Availability of modular equipment (from 50 to 10,000 m3/day per unit),.
As a result of implementation of the fast clarification technology with application of weighting agents and usage of thin-layer modules in settling tank with recirculation of microsand, the efficiency of water precipitation and clarification is increased.
Application of AS sorption filtering bed allowed to achieve stable total iron content in water of less than 0.3 mg/l.
Fully automated process control system allowed to optimize maintenance expenses.
Implementation of on-line control of qualitative characteristics of “raw” and treated water allows to quickly react and make adjustments to the chemical dosing system.