Use of Combined Installations for High Sulfides and Hydrogen Sulfides Content Sewage
Up to 1970-s in the world practice of sewage treatment plants constructing biological filters having a reliable technological mode of operation and low power consumption dominated. However, experience has shown that their most stable work results in oxidation of organic matter for 50-70%. Existing aeration facilities have the following disadvantages: significant energy consumption (0,4-0,6 kWh per 1 m3 of urban waste water); unreliability of blowers, high pressure fans, mechanical aerators in the long-term operation.
In countries with hot climate temperature of processed liquid rises above 30oC, resulting in decreased solubility of oxygen in the liquid and reduced rate of biochemical treatment. In towns and cities of Easten countries residential areas with low levels of improvement and lack of a centralized sewage make high proportion. Wastewater is collected in septic tanks and cesspools, where process of biodegradation flows intensively with the formation of reduced forms of sulfur - sulfides and hydrogen sulfide concentrations above 30 mg/dm3 and ammonia nitrogen over 50 mg/dm3. When this wastewater is delivered to centralized sewer overflow stations and due to the decay processes of organic pollutants in the receiving tanks and reservoir, the total content of sulfides and hydrogen sulfide is 15-40 mg/dm3, ammonium nitrogen to 50-80 mg/dm3. It is known that hydrogen sulfide is a poison and its enzyme content in waste water fed to the biological treatment should not exceed 2 mg/dm3. When values of ammonia nitrogen is basic its content in treated water after biochemical treatment processes with nitro-denitrification exceeds 10 mg/dm3, which is higher than limited standards-2 mg/dm3, adopted in many countries. Organizations engaged in wastewater treatment, restrict the admission of rotting concentrated wastewater, so that they are disposed to landfills and unauthorized places. Therefore this problem deserves serious attention.
Since 1982, in Rostov Scientific Research Institute of the Academy of Public Services according to state orders, a system of new types of wastewater treatment plants for towns, cities and enterprises producing and processing agricultural products has been developed.
The central link of the system is combined works (CW), with signs of biofilters and aerotank [1].
In CW jet aeration system making the work more reliable and cost-effective is implemented.
Oxygen saturation of the treated waste water is carried out:
- when air oxygen dissolves in the water during the irrigation with a mixture of wastewater and sludge of biofilter loading
- as a result of mass exchange transfer at the end of a film of fluid over the surface of plane biofilter loading
- a saturation of the treated water in the aeration basin by atmospheric oxygen at the expense of additional oxygen dissolution in aeration columns and by lifting the air bubbles.
The principle of jet aeration is carried out by CW at a certain level of the liquid over the tops of the columns cuts, and certain ratios of column height above the liquid surface to the recessed part. In this case, there are vortex funnel, involving air pipes into the holes.
CW (picture 1) represents ideal mixing reactors : waste water initially mixes with activated sludge in the mixing chamber, then it contacts with biofilter biocoenosis, then it is equally introduced and mixed with the entire volume of the aeration zone, and finally filtered through a layer of floating scum in sediment zone.
CW construction eliminates slippage of untreated wastewater and provides high quality cleaning. The overall organization of wastewater treatment process, implemented in biofilter-aerotank, allows the process of sewage treatment in the extended aeration mode, designed for complete oxidation of the organic impurities, partial mineralization of the biomass and nitrification.
Oxidation of organic pollutants in wastewater in biofilters is done by immobilized microflora. CW biofilters working terms are specific and differ from the conditions in which the existing biological filters work: biofilter loading is irrigated not by clarified waste water, but by mixture of wastewater and sludge, the concentration of suspended solids, thus, is 3-6 g/dm3; hydraulic load is 2-3 times higher than in traditional high loaded biofilters and loading irrigation is done incessantly. Not only the processes of sorption and degradation of organic contaminants with the achievement of a higher degree of purification take place in CW biofilter provide, but also well developed processes of nitro-and denitrification.
A new stationary irrigation system of biofilter loading irrigation with trays, drain pipes and reflective disks, based on the incidence and impact of jet fluid on the solid surface was specially elaborated for CW. While hitting the jet is divided into many drops with different trajectories of falling.
In the existing aeration plants used in countries with hot climates, due to direct sunlight and high temperature of air supplied to the liquid aeration, temperature of treated water significantly increases. Implementation of combined facilities in the enclosed design (with sandwich panels) partially solves the problem of overheating of the liquid. Features of constructive devices allow CW to bring the ratio of oxygen to 20% (theoretically possible to use up to 30%) due to its repeated circulation together with the treated liquid, ie reduce the number of outdoor hot air for the biochemical processes in 3-5 times [2].
Optimal operation of aerobic biological treatment plants depends on a large number of regulated and unregulated factors, in particular, the presence of reduced sulfur compounds. Today, more and more we can notice the exceeding of hydrogen sulfide concentration in the incoming wastewater. This is true for sewage from small villages, which are characterized by the presence of septic tanks. However, for large settlements there is also a trend of increasing concentrations of hydrogen sulfide, especially at high extent of sewerage systems, anaerobic conditions which provoke sulfate. The concentration of sulfates in wastewater, usually corresponds to the concentration in the natural. And now there is a tendency to increase sulfate in natural water source. For example, the content of technogenic sulfate ions in the river Don for the past 60 years has increased by 4,7 million tons. In the mechanical treatment plants in the absence of oxygen the process of sulfate reduction continues, as in biodegradation of the protein under anaerobic conditions there is an additional intake of sulfur-containing compounds. The relationship between molecular soluble sulfide and hydrosulfide depends on pH of water. At pH values common to most municipal wastewater (pH 7.5) mainly contains hydrosulfide (70%). Under aerobic conditions the oxidation of sulfides to sulfates is done by thio-organisms, high concentration of which adversely affects the overall performance of purification. In particular, thiobacteria microorganisms are very competitive in the consumption of oxygen in relation to other groups of microorganisms, and the priority becomes the process of oxidation sulfur compounds. In addition, sedimentation properties of activated sludge change, and the process of sludge swelling is observed. Thus, the occurrence of such conditions leads not only to complicate the operation of sewage treatment plants, but also to a deterioration in the quality of treated water.
Features of CW constructive device can reduce the negative impact of hydrogen sulfide and sulfides on the biochemical processes of wastewater treatment. First of all, in the technological scheme with combined facilities are no primary settling tanks, which reduces the likelihood of further sulfate.
In the irrigation system and the biofilter with in-plane load within a film flow of liquid on the surface a transition of hydrogen sulfide in the atmosphere according to its partial pressure occurs. In addition partial oxidation of sulfides immobilized microflora, which lowers the burden of sulphides in activated sludge aeration tank occurs in biofilter.
Water-jet aeration system used in CW, enables flexible management of dissolved oxygen concentration in the interstitial fluid aeration tank, which, along with a high rate of air compared to other aeration systems, reduces the negative effect of sulfide on the process of wastewater treatment. These features of CW, as shown by production testing, allows high-quality treatment for all indicators in the initial content of reduced sulfur compounds to 8 mg/dm3, which is the undoubted advantage of CW in comparison with classical aeration tank.
Sewage treatment station in Kushchevskaya village, Krasnodar region (capacity 2,7 thousand m3/day), (picture 2) can be shown as an example of CW exploitation in the regime of high loads of hydrosulfide and hydrogen sulfide.
Sewage treatment plant in Abrau-Durso village, Krasnodar region.
In January-March 2009. at initial waste water temperature of 7-10oC and residence time in drainage system 10-20 hours, sulphides and hydrogen sulphide content increased to 5.3 mg/dm3; in April-June at at initial waste water temperature of 13-16oC, outdoor air temperature 15-32oC content of sulphides and hydrogen sulphide increased to 30-40 mg/dm3. Accordingly, condition of activated sludge became worse-it darkened, purified water had a gray color with milky appearance. Water clarity according to Snellen font decreased to 6-8 cm. But even with all that events microflora of biofilter and aeropack remained active: the content of organic contaminants in treated water after CW was on the BOD 12-20 mgO2/dm3; on COD 18-40 mgO/dm3. However, high concentration of hydrogen sulfide and sulfides adversely affected the process of nitrification. The content of ammonia nitrogen was reduced from 75 mg/dm3 only up to 28 mg/dm3. With cessation of cesspit content supply the content of sulphides and hydrogen sulfide was reduced to 8-20 mg/dm3, and improvement of wastewater treatment quality began (transparency 16-22 cm, according to Snellen font). The observed phenomena helped to establish the maximum allowable concentration of reduced sulfur compounds in the incoming wastewater to CW: with nitrification-8 mg/dm3, in the absence of nitrification-15 mg/dm3.
To eliminate the negative impact of hydrogen sulfide and sulfide (more than 8 mg/dm3) on the work of CW studies were conducted to remove reduced sulfur compounds by physical and chemical methods.
Of all the reagents used in wastewater treatment, the most appropriate was the use of hydrogen peroxide, which is a selective oxidant with respect to hydrogen sulfide and sulfides and have chemical inertness to ammonium compounds.
As a result of our research the estimated dose of hydrogen peroxide was determined (DH2O2):
DH2O2 = 1.38 (K1 * CHS-+ K2 * CH2S)
where: 1,38-Technology coefficient;
K1 and K2-stoichiometric coefficients;
CHS- and CH2S-concentrations of hydrosulfide and hydrogen sulfide in the source water.
An additional positive feature of hydrogen peroxide is sharp increases of oxygen concentration while its introduction to the treated water that must be considered when choosing the point of reagent entering.
Another way of solving this problem is to use a two-step purification scheme with combined structures (picture. 3).
Wastewater after preliminary mechanical treatment (separation of coarse sediment and sand) are sent in the first stage of CW (CW-1) in which, at high loads on immobilized biofilter microflora and active biomass in aeropack on reduced sulfur compounds are separated hydrogen sulfide and oxidation of the basic mass hydrosulfides occurs. Time of wastewater spent in CW-1 varies within 1-3 hours. Next wastewater undergo full biological treatment at CW of the second step (CW-2) at low loads of sludge organic matter (0,05-0,1 kg BOD per 1 kg of ashless substance) with the period of stay 7-10 hours. Technological scheme provides filing of initial wastewater into the mixing chamber of CW-2 and the pumping of sludge into the mixing chamber of CW-1, which allows flexible management of purification process during changes of hydrogen sulfide and hydrosulfides content and sharp fluctuations of hydraulic and organic loads. In this case, favorable conditions for the processes of nitrification and partial denitrification are created.
The system of new types of sewage treatment facilities ensures the reliability of technological modes of treatment, high and reliable quality cleaning, lower capital and operating costs (energy consumption lower by 2-3 times, reduction of staff by 50%). New types of compact and environmentally friendly sewage treatment plants can be installed directly in the neighborhoods (picture. 4).
(CW 2).
But in this case, not only the disinfection of used air, but its degassing and deodorizing occurs.
For the purification of concentrated and highly concentrated wastewater, and if necessary nutrients removal, technological scheme can be supplemented by biocoagulators, anaerobic and aerobic reactors, with the use of specific constructive solutions, facilities for physical-chemical treatment of wastewater.
The main task now is to update the existing treatment facilities. Renovation works according to technology of RSRI APS with full use of previously built structures increases the efficiency and profitability of sewerage systems. Refund loans by reducing operating costs is possible over 5 years.
References:
[1] V.P Kolesnikov, E.V Vilson, V.K Gordeev-Gavrikov “Combined works with biofilters and aerotanks”, magazine “ZHKH” ¹ 12, Part I., Moscow, Russia, 2003.
[2] V.P Kolesnikov, E.V Vilson “Modern development processes of sewage treatment plants in combined works” publishing house "South", Rostov-on-Don, Russia, 2005.