The modern wastewater treatment infrastructure in cities is in many cases in poor condition, while the volume of wastewater and the concentration of contaminants keep growing. The quality of wastewater treatment may fall short of standards due to unsatisfactory operation of the aging treatment plants. Besides, new sewage lines must be built through residential area to supplement the existing infrastructure. The larger the treatment facilities, the mode costly are maintenance, repairs and operation, which becomes so cumbersome that the expenses keep growing indefinitely.
Another unpleasant issue is that with the growth of aeration facilities, the amount of bio sludge and the area of the facilities grow proportionately. In large cities treatment facilities may take up as much as 1000 hectares.
Compact treatment facilities can help avoid these drawbacks, while using new technology allows to improve treatment quality to meet the highest standards.
Biochemical methods, apart from mechanical, chemical and biological, are considered the most promising.
Three types of bioreactors are used in local systems for deep wastewater treatment: biofilters, aerotanks and sewage farms. Among them are biogilters with sand and biofilm, used in the US, the UK, Japan and France.
The gravity feed of wastewater in combined biofilters eliminates the need for pumping stations and separate buildings with heating and lighting.
High parameters of wastewater treatment may be received by a compact system with submerged biosludge membranes, which consists of separate anaerobic-aerobic treatment chambers, due to the passage of wastewater from the bottom to the top through the biomass. This technology also allows to produce biogas.
Another compact option with good treatment quality are two coaxial polyethene cylinders. The ring between the cylinders is separated into zones by radial screens, and wastewater flows through them consecutively. The various facilities combine various levels of treatment: destruction of arganic compounds, nitrification, denitrification, microfiltration etc.
Biochemical treatment of household wastewater is performed in vertical rectangular tanks, with a four sided pyramid in it. The space between the rectangle and the pyramid is the aeration zone, while the space in the pyramid is the secondary settling tank. Bio sludge returns to aeration zone through four slots in the bottom.
Operation of small treatment plants is prone to sharp variations in flowrate and contaminant concentration. To compensate for this and to ensure a more even supply of water for treatment, an equalization tank is recommended. Wastewater from several buildings can be treated in a single structure of several concrete modules. The first module performs as a collector and settling tank, the other performs aerobic oxidation of organics and ammonia nitrogen, the third module performs denitrification. Excess biomass is supplied into the first module for stabilization with primary sediment.
A final treatment section can be added to the system for better treatment, where water passes through layers of filtering material with decreasing mesh. The processed water is decontaminated by UV-light.