Renovation of the Sestroretsk treatment facilities with deep biological P removal
The process methods of biological dephosphorization have been designed and are quite widely applied in effluent treatment. The principle of biological removal of phosphorus is based on living activities of microorganisms, particularly acinetobacter that are able to accumulate more phosphorus that is required for growth – this is the so-called “voracious absorption”. Acinetobacter (phosphate accumulating organisms – PAO) are usually present in active sludge, but in insignificant amounts. For these microorganisms to start playing their useful role, it is necessary to provide them with low molecular volatile fatty acids (VFA), which serve as a substrate for them, and to create conditions under which they are able to use VFA more efficiently than other microorganisms, which are in biocoenosis.
The applied methods of biological dephosphorization, however, allow removal of total phosphorus during biological treatment of domestic wastewater only up to concentrations of about 1 mg/l. A deeper removal of phosphorus is achieved by using chemical coagulants. This method, however, requires considerable coagulant consumption, which significantly increases the cost of effluent treatment and facilitates formation of an additional volume of sediment with a high content of metals, and also leads to secondary contamination of the effluent being treated. Upon upgrading a method of additional aging was used – “maturation” of active sludge in the zone of the biological unit of the aerotank oriented at deep removal of nitrogen and phosphorus
Components installed in the solution
The developed technical solution can be implemented at the existing structures of biological active sludge effluent treatment.
The method of biological effluent treatment by active sludge, providing for the presence of sludge slurry in the anoxic, aerobic and anaerobic zones with the purpose of deep removal from sewage water, uses the process method of ageing of active sludge in the aerotank with deep biological removal from sewage water of organic substances and biogenic elements – phosphorus and nitrogen. It is distinguished by the anaerobic zone being combined with the process of sediment fermentation (obtaining of volatile fatty acids), and there is no forced mixing of active sludge in this zone. This zone can be called a “maturation” zone. In view of there being no mixture, the heaviest fractions of active sludge descend to the bottom and are condensed up to the concentration of about 40 g/l. The oxygen content in the condensed deposits drops to the values 0.2 – 0.1 mg/l, of nitrates – to the values below 1 mg/l. At that, phosphorus-accumulating bacteria use polyphosphates accumulated in the cell, and the content of phosphorus in the culture liquid increases dozens and hundreds times (from tenths of mg/l to 50 – 150 mg/l). Concurrently, there is fermentation of organic substances, active sludge (deposit) to volatile fatty acids. When sludge (deposit) is at the bottom of the “maturation” zone for several days, the gas emitted in the process of fermentation lifts the condensed sludge to the surface, and it is carried by flow from the “maturation” zone. This process, deposition of part of the incoming sludge to the bottom and floating of the fermented sludge, occurs continuously. To prevent methane fermentation, short-term supply of air to the “maturation” zone or a forced removal of deposit are possible. The “maturation” zone can be set up in primary settling tanks as well. The advantages of this method are:
- considerably longer (several days) presence of heavy fractions of sludge in the anaerobic “maturation” zone compared to the presence of liquid (several hours). This allows a deeper fermentation of deposit onto fatty acids with a smaller volume of the anaerobic “maturation” zone, as well as creation of more advantages for phosphorus accumulating bacteria – as a result, a deeper removal of phosphorus from effluents being treated.
- no expenses for pumps and electric power to organize anaerobic circulation of sludge slurry.
- opportunity to maintain a wider spectrum of the composition and age of the sludge in an aerotank, which allows better alignment of the processes of nitro-denitrification and dephosphorization.
The use of UCT (University of Cape Town) technology at Sestroretsk Sewage Treatment Plant makes it possible to reach the new HELCOM recommendations for total phosphorus of 0.5 mg/l only when using a coagulant – in the amount of 30–35 mg/l (by Ferix-3).
Arrangement of the “maturation” zones in the beginning, i.e. a substitution of the technology for UCT-K makes it possible to reach the new HELCOM recommendations for total phosphorus of 0.5 mg/l without using a coagulant and with a sufficiently large margin after auto-selection of active sludge biocoenosis.
Determination of the content of phosphorus of phosphates and VFA in the filtered samples of wastewater coming into the aerotank and culture liquid directly in the “maturation” zone has shown that at the entry of the aerotank of phosphorus of phosphates was 1.2 mg/l; in the “maturation” zone – 155 mg/l; VFA respectively – 1.6 and 4.6 mg-eq/l.
Thus, not only phosphorus release, which indicates active living activity of PAO (phosphate accumulating organisms) takes place in the “maturation” zone, but also acidofication, supplying VFA directly to the zone of their accumulation by phosphate accumulating organisms with the use of energy of polyphosphate links. At that, unlike traditional acidification methods, no allotment of additional equipment for preliminary acidofication of raw sludge is required.
The advantages of the method are:
- considerably longer (dozens of times) presence of sludge containing phosphate accumulating bacteria in the anaerobic “maturation” zone compared to the time of presence of liquid. This allows, with comparatively small volumes of the anaerobic zone, the prolonged presence of the phosphate accumulating bacteria under anaerobic conditions.
- possibility to perform deep hydrolysis and fermentation of deposit directly in the anaerobic zone – the zone of presence of the phosphate accumulating bacteria. At that, supplying precisely this group of bacteria with fatty acids.
- no expenses for pumps and electricity to organize anaerobic circulation of sludge slurry.
- due to a subdivision into light and heavy fractions of sludge flakes, there is an opportunity to maintain a wider spectrum of the composition and age of the sludge in the aerotank, which allows a better alignment of the processes of nitro-denitrification and dephosphorization.
- an opportunity to stop the transition of the process into methane fermentation in good time by supplying air to the “maturation” zone through aerators installed on the bottom of the zone.
- upon shock discharges, it is possible to promptly increase the operational concentration of sludge in the aerotank by short-term supply of air to the “maturation” zone and agitation of part of the condensed sludge. Introduction of this method at biological treatment facilities allows the concentration of biogenic substances in treated domestic wastewater to be reduced to the standard values (MAC in a fishery water body) – 0.2 mg/l by phosphorus of phosphates.