System for sorption removal of toxic substances from water using powdered activated carbon
To ensure safety of water supply while detecting toxic substances in water at the source, the State Unitary Enterprise "Vodokanal of St. Petersburg" launched a system for sorption removal of toxic substances from water using a system of powdered activated carbon (hereinafter referred to as PAC) dosing.
This system can ensure the removal of substances accounting for the smell of water, oil products, or toxic substances penetrating water at the water source due to man-caused accidents or acts of terrorism.
The powdered activated carbon (PAC) is used, as it is necessary to remove odorant substances by a sorption method (2-methyl-isoborneol 2,4,6-trichloro-anisol 1-geosmin, 2-geosmin), water-soluble oil products (with daily registration of their concentrations in the water of the water source in excess of PAC (0.1 mg/dm3) and toxic substances provided that:
- the presence of the above contaminations has a periodic short-time nature;
- the treatment at strainer facilities and contact flocculators is not efficient.
Components installed in the solution
An automatic station for PAC preparation and dosing consists of the following functional assemblies:
- Big bag emptying unit. The unit consists of a lifting device with an emptying system. All equipment of the unit is manufactured in an explosion-proof design.
- Activated charcoal storage unit consists of a (silage) storage hopper, as well as a system of pneumatic conveying of coal and a screw dosing conveyor. A PAC tossing device is provided to prevent caking. PAC is conveyed with a help of a compressor and an injector for dry substance. All equipment is executed in an explosion-proof design.
- Preparation and dosing unit is installed in a container with a heating, ventilation, and air conditioning system. The slurry is prepared by the following principle: the set quantity of PAC is supplied to a known quantity of water. Coal slurry is prepared in a tank with maximum concentration 10%. Dosing pumps dose the PAC solution to raw water conduits proportionately to the raw water consumption in each water conduit. The flow of the PAC solution is controlled with flow meters.
In 2017, the coal facilities of the Main Water Supply Station in St. Petersburg were upgraded ensuring the wet storage of PAC. An increase in the efficiency of sorption treatment is ensured by using hot water at the stage of preparation of the initial slurry in the wet storage system.
The stations for powdered activated carbon dosing have high-technology equipment, the process is fully automated. To ensure uninterrupted and efficient operation of the system, technical works have to be systematically performed by qualified personnel to ensure faultless operation of equipment. An agreement with a contracting organization is concluded annually to perform works of maintaining equipment of the system of PAC receipt, preparation and dosing.
All water intake facilities of water supply stations designed for drinking water production from the surface water sources of the Neva River and the Gulf of Finland in Saint Petersburg are equipped with systems for PAC receipt, preparation and dosing. The systems were commissioned in 2005 at Kolpino Water Supply Station, North Water Supply Station, South Water Supply Station, Volkovo Water Supply Station, Main Water Supply Station, Waterworks of Kronstadt.
The technology of using PAC at the water treatment stage at the waterworks operated by SUE "Vodokanal of St. Petersburg" allows organoleptic and water safety indices to be ensured in case of a threat of toxic substances in the surface water source.
This technological solution is not used continuously. Normally, PAC is dosed relatively rarely and for short periods of time. PAC is only applied when the water quality at the source decreases with appearing oil products (concentration more than 0,15 mg/l over 2 days) or smell. With the oil products concentration of 0,15-0,2 mg/l the dosage of PAC is 3 mg/l. Dosing of PAC stops when the oil products concentration does not exceed 0,1 mg/l over 2 days.
Applying PAC makes it possible to:
- efficiently remove organic substances – odorants generating unpleasant flavor and odor of water, and thus improve organoleptic properties of water;
- efficiently remove oil products and toxic substances, if their concentrations in the source water is registered.