Effective sewage treatment

V.P Kolesnikov, E.V Vilson, VA. Vavilin, N.V Kolesnikova

In Rostov Scientific Research Institute of the Academy of Public Services a system of new types of wastewater treatment plants for towns, cities and enterprises for agricultural products production and processing was creating. It is called CW (combined works) and allows purification of waste water with a concentration of contaminants within BOD up to 1000 mg/l and suspended solids up to 400 mg/liter. The design parameters of the combined structures are defined by their performance. According to this indicator we distinguish CW 25-100 m³/day; 200-100000 m³/day. Technological and constructive features of CW were first presented at the Russian-German symposium in February 1995 in Berlin. CW was awarded by Gold medal at in 45th International Inventions Exhibition in Brussels in 1996 for its parsimony. For treatment of high concentrated sewage as well as for deep nitrogen and phosphor cleaning, technological scheme can be supplemented by anaerobic bioreactors, biocoagulators, physical-chemical treatment structures. The novelty of solutions and operating of structures are approved by more than 20 Russian patents. Nowadays patents’ registration is in process in the U.S., Canada, Australia, Europe.
The main feature of the CW is its constructive integration of biofilter with in-plane loading and aeropack into a single unit. Also air from water-jet aeration system is used for aerotank saturation with oxygen. Picture 1 shows a scheme of CW operating.

Waste water after mechanical treatment comes into the mixing chamber, where it mixes with sludge fluid coming from aerotank. The mixture of wastewater and sludge fluid goes to water-distributing trays of biofilter irrigation system with the help of circulation pumps. Biofilter dimensions are appointed on the base of conditions of 50-80% degree of wastewater purification. Purified water is discharged into the collecting tray from where it enters the aeration columns of aerotank. While pouring the liquid into the aeration columns vortex funnels appear and air entrainment occurs. While air-water streams motion, dissolution of atmospheric oxygen and mixing of aerotank contents occurs. Circulating pump performance, the amount of aeration columns, their diameter, height above and below the water level are appointed on the base of aerobic microorganisms of activated sludge in aeration tank oxygen supply. Structural dimensions of aerotank are determined by the condition of achieving a given degree of purification. Sludge mixture is displaced from aeratank and enters the sediment zone, where the sludge is grouped into flakes and settles, and the purified water is drained through a collecting tray into post-treatment construction. In the combined works apart from the processes of organic pollutants biodegradation, the processes of nitrification and denitrification occur: in biofilter processes of organic contaminants biodegradation and denitrification predominate, and in aerotank - processes of biodegradation of organic matters and nitrification predominate, which is possible due to the low load on the sludge on the organic matter.
Application of artificial regular loadings in biofilters with high density of the working surface of 100-150 m²/m³ and constructive solutions, providing effective biocenosis appearance on the surface under high hydraulic loading 70-150 m³/m²day and excluding loading flooging under concentration of sludge in the circulating fluid up to 7 g/dm³, allowed maximum use of the inherent biofilter mass-exchange properties and to increase their oxidative capacity.
Dimensions of biofilters are determined on the basis of calculations and structural considerations: the basic dimensions of facilities, size of sheets and passages.
The effect of treatment in biofilters in fractions of a unit is calculated by the formula:

where:

-Average amount of biomass formed per 1 cm² artificial loading surface, dry matter (according to RSRI APS), = 0.007 g/cm²;
V-Average oxidation speed of in g BOD. at 1 g of biomass at dry matter per day for municipal sewage and wastewater calculated temperature of sewage in winter time T = 10^oC,
V = 0,27-0,31 g BOD./g.day;
F-Surface area of the download sheets in one section, cm²;
m-number of sections in the combined works;
q- Sewage flow rate, m³/day;
La-BOD of the initial waste water, g/m³;
Lt- BOD of treated waste water is taken as 10 g/m³.
Calculation of reactionary volume in aerotank is equal to determination of the amount of aeration tanks and the amount of suspended silt in the sediment area.
Determining the amount of one section is performed by the formula:



Where:

t-Time of sewage with sludge contacting;


- Dose of sludge in g/dm³, for household and close to them wastewater, it is recommended to count as 4 g/dm³;
- Average speed of complete oxidation of contaminants in BOD₅ per 1 g of dry matter in 1 hour for calculated temperature of household waste T = 10^oC, was adopted within 2,4-2,9 mg/g.ch. For other wastewater temperature and composition according to experimental studies.
In determining the reactionary volume of sediment zone, the boundary of suspended sludge is appointed at level not less than half of the zone height.
When projecting the aeration system it is necessary to take into account the intensity of oxygen saturation of sludge and the intensity of the turbulent regime in the area. On the base of these conditions the multiplicity of effluent circulation and performance of the circulation pumps is determined .
Water jet aeration centers are to be performed in one or more bundles.
Calibration of oxygen balance calculation is performed by the formula:

where:

-Specific consumption of oxygen in the aeration basin, working on a method of complete oxidation, assumed to be equal to 1.1 gO₂/gBOD;
OCk–Oxidizing capacity of aeration columns, gO₂/h, is determined by the schedule of RSRI APS;
–Coefficient, taking into account the saturation of the liquid oxygen in the biofilter. It is determined by the schedule of RSRI APS;
-Coefficient of columns angles impact on OCk, is determined by the formula:



Where:
-Inclination angle in units;
C₁–Oxygen content in treated water is assumed to be 2 g/m³;
C₂–Oxygen concentration in the sludge mixture from the aeration tank into the mixing chamber, is taken equal to 1,5 g/m³;
Z-Number of aeration columns, pieces.;
n–Adopted by the multiplicity of circulation, which is then adjusted for size differences between the necessary and actual intake of oxygen.
Since 1985, CW operates on a number of new and reconstructed buildings of Moscow region, Krasnodar, Ulyanovsk, and Rostov regions. During the operation period from 1985 to 1998, the degree of purification of BOD₅ was not less than 98%, while the concentration of organic contaminants on BOD₅ in treated water was 7-15 mg/liter. When adding into the scheme the after-treatment system (aerobic bioreactor with artificial load) the content of organic contaminants decreases within BOD₅ up to 3 mg/l, for suspended solids up to 2-3 mg/liter.
Experience in CW construction and operation has allowed to prepare design solutions for the reconstruction of inefficient sewage treatment plants of various designs. The reconstruction of existing treatment facilities by the proposed method ensures: receiving high quality treatment, both in warm and in cold seasons, and increasing productivity of treatment facilities at the existing premises in 1,5-3 times; reduction in specific energy consumption by 2-3 times; reduction of staff by at least 30%.
CW Implementation in the form of closed structures allows to organize the removal and processing used in the biochemical processes of air, which provides environmentally friendly mode of treatment facilities. Sewage treatment plants, according to the proposed design can be placed directly in residential neighborhoods.