bioscrubbling of drain waters

From Big Medical Encyclopedia

BIOSCRUBBLING of drain waters — the way of release of a liquid phase of drain waters from organic matters based on use of the oxidation-reduction processes proceeding with the participation of microorganisms.

As a result of B. of the lake the carbohydrates and fats which are contained in drain waters break up under the influence of microorganisms and their enzymes to water and carbonic acid. Proteinaceous molecules are split through proteoses and peptones to amino acids. A part of amino acids is used as plastic and power material by the breeding microorganisms, and other part is exposed to deamination (see) with formation of ammonia and acids of a fatty and aromatic series. In aerobic conditions organic to - you are oxidized to carbonic acid and water, and ammonia communicates carbonic acid, forming ammonium carbonate.

Nitrogen-containing organic matters get to drain waters in the form of products of exchange, in particular in the form of urea, edges under the influence of urobakteriya is hydrolyzed with formation of ammonium carbonate. Further ammonium carbonate is exposed to biochemical oxidation by means of the aerobic bacteria from the sort Nitrosomonas and Nitrobacter for the first time allocated in 1890 with the Russian scientist S. N. Vinogradsky.

This process which received the name of a nitrification takes place in two phases. In the first phase of biochemical oxidation ammonium salts turn into nitrogenous compounds (see. Nitrites ) coccal bacteria from the sort Nitrosomonas, and in the second — in nitric (see. Nitrates ) bacteria of the sort Nitrobacter. The course of reaction according to S. N. Stroganov:

(NH 4 ) 2 CO 3 + 3O 2 ⇆ 2HNO 2 + CO 2 + 3H 2 O + 148 kcal.

2HNO 2 + O 2 ⇆ 2HNO 3 + 44 kcal.

Nitric to - that in the form of mineral salts (nitrates) is an end product of oxidation of proteic matters and products of their exchange in an animal organism. Therefore by amount of nitrates judge efficiency of bioscrubbling. In the course of a nitrification heat which is used at operation of constructions of bioscrubbling of drain waters during the winter period is generated, and the reserve of oxygen which can be used for biochemical oxidation of organic nitrogen-free matters when all free (dissolved) oxygen is completely already spent collects. Under the influence of the denitrifying bacteria opened by the Russian scientist V. L. Omelyansky, oxygen is chipped off from nitrites and nitrates and for the second time is used for oxidation of organic matter. This process is called a denitrification, under a cut understand recovery by bacteria of salts nitric to - you (nitrates) irrespective of whether salts nitrogenous to - you (nitrites), the lowest nitric oxides, ammonia or free nitrogen are formed at the same time. At the same time in alkaline condition and at broad access of air recovery process does not go further formation of salts nitrogenous to - you; and in acid medium and at the complicated inflow of oxygen recovery goes to ammonia.

A denitrification in narrower word meaning call decomposition of nitrogen-or nitrite salts with release of free nitrogen. Without having free oxygen or having it in limited quantity, denitrifying bacteria take it from salts of nitric and nitrogenous acids and at the same time oxidize nitrogen free organic compounds, scooping energy necessary to them in this oxidizing process. This complex at the same time recovery and oxidizing process can be (according to V. L. To Omelyansky) it is presented in the form of the equation (where With — organic carbon):

5C + 4ΚΝO 3 = 2K 2 CO 3 + 3CO 2 + 2Ν 2 .

Therefore, at B. of the lake of drain waters along with oxidizing also recovery processes of a denitrification at which microbes consume oxygen of the formed nitric connections proceed. This process is important in an initial stage of sewage treatment on all types of treatment facilities and especially for neutralization of the organic matters which got with drain waters to deeper layers of earth. In process of accumulation of oxidates and saturation of a drain water free oxygen recovery processes are slowed down.

The main objective of B. of the lake — as much as possible to exempt drain waters from organic matters, to reach a high level of relative firmness (stability) of waste liquid, at a cut is considerably weakened or its ability to rotting is at all lost. Relative firmness, or stability, drain waters is expressed as a percentage and makes the relation of amount of the oxygen which is contained in a drain water in the state dissolved and connected with nitrites and nitrates, to quantity it, necessary for biochemical oxidation of all organic matters. The size of firmness determines terms of rotting of liquid. So, at firmness of 50% and t ° 20 ° rotting begins for the third day, at firmness of 80% — for the seventh day, at firmness of 99% — for the twentieth day, at firmness of 100% water does not rot. At a temperature less than 20 ° firmness increases. Firmness of a raw drain water usually is less than 11%, and after full B. of the lake it shall raise not less than to 99%. In some cases depending on sanitary, hydrological, climatic and other conditions the production of sewage in a reservoir with relative firmness of 80% is allowed.

For protection of reservoirs against pollution of a condition of descent of drain waters shall be considered specifically on each reservoir. To define conditions of production of sewage in a reservoir — it means to calculate what concentration of pollution is allowed in a certain quantity of the lowered sewage in the known reservoir not to cause pollution of a reservoir or not to violate requirements imposed to quality of water of a reservoir «Rules of protection of surface waters from pollution by drain waters» (see. Sanitary protection of reservoirs ).

Speed, completeness of disintegration and a mineralization of organic matters at B. of the lake depend on a number of conditions: the mass of the microorganisms participating in B. of the lake, extents of providing air with their oxygen of quality and amounts of the organic matters arriving with waste liquid of temperature of drain waters, and also pH of the environment, presence of chemical substances, etc. B. the lake of drain waters most successfully occurs under favorable conditions (pH of drain waters within 6,5 — 8,5, temperature not lower than 6 and not higher than 30 °, the general concentration of the dissolved salts no more than 10 g/l, concentration of chemical substances within the sizes which are not influencing processes of self-cleaning) and existence of the minimum maintenance of biogenic elements (nitrogen of ammonium salts not less than 15 mg/l and phosphates not less than 3 mg/l). The task of the technical organization of this process comes down to creation of such conditions under which organic matters of drain waters will be given to contact with aerobic microorganisms and oxygen of air, and their disintegration will go with perhaps bigger completeness and speed. For this purpose B.'s constructions of the lake of drain waters which are subdivided into two main types serve: 1) constructions in which B. the lake occurs in the conditions close to natural and 2) constructions in which cleaning happens in simulated conditions. Treat the first type of constructions fields of irrigation (see), fields of filtering (see), the platforms of underground filtering filtering the trenches with a natural layer of earth filtering wells and biological ponds. To the second type of constructions the aerotanks, biofilters, peschanograviyny filters filtering trenches with a bulk layer of earth and installations on full oxidation of drain waters treat (circulating oxidizing channels, oxidizing blocks, compact installations, aerotanks with the prolonged aeration in other).

Biological (oxidizing) ponds represent artificially created reservoirs in which B. the lake proceeds in conditions, the closest to the natural course of self-cleaning of reservoirs. Distinguish aerobic and anaerobic biological ponds which size usually no more than 1 hectare, depth of aerobic ponds no more than 1 m, and anaerobic ponds — not less than 2,5 m. The following types of aerobic biological ponds are applied.

Flowing ponds with dilution of waste liquid. The waste liquid clarified in settlers is diluted by 3 — 5 times with a river water. Time of stay of the purified waste liquid in a pond makes 8 — 12 days, loading of waste liquid makes from 125 to 300 m 3 /га in days.

A lack of such ponds is need to build preliminary sedimentation tanks and to construct a dam for the device of a pure pond tank.

Flowing ponds without dilution of waste liquid were for the first time established at the initiative of S. N. Stroganov on the Moscow fields of filtering.

Waste liquid after preliminary upholding passes a series (4 — 5) consistently reported ponds during 30 days. In the first pond microorganisms of a polysaprobic zone, in the second and third — alpha mezosaprobnoy, and in the fourth — a β-mezosairobny zone prevail. Biological ponds prior to the beginning of a running cycle shall «ripen» biologically (within a month during the summer period). Loading on BPK (see) on the first pond (at three and more ponds) the equal 250 — 300 kg/hectare a day are accepted; average loading on BPK depending on water temperature — 50 — 75 kg/hectare a day.

Contact ponds. Waste liquid moves in a series in parallel of the located landlocked tanks. In a dead water processes happen in an expedited manner.

For a midland time of contact varies within 8 — 10 days, for the southern areas — 5 days. Productivity of contact ponds is 1/2 — 2 times more, than flowing.

Since 1950 in Minsk are observed over landlocked biological ponds in which self-cleaning of waste liquid is connected with mass development in them the most widespread species of seaweed of Chlorella.

As a result of photosynthesis of an alga, acquiring carbon from carbonic acid, oxygenate and supersaturate water. Receipt in water of oxygen as a result of reaeration in these conditions is mattered by smaller value. In such ponds intensive oxidizing processes develop, and in 8 days there comes almost full mineralization of organic matters of drain waters and death of pathogenic microflora.

When the high effect of sewage treatment before their release in reservoirs is required, previously purified drain waters on constructions of artificial bioscrubbling are exposed to after-treatment in biological ponds.

Estimated time of stay usually is accepted equal 3 — 5 days. Load of ponds is accepted taking into account their aeration, edges are made by 6 — 8 g of oxygen on 1 m 2 pond. It is enough to provide after-treatment of 4000 — 5000 m 3 /га drain waters in days.

The main role of oxidizers and mineralizers of organic matter of drain waters in aerobic ponds is carried out by plankton consisting of bacteria, seaweed and the elementary organisms. So, the quantity of rotifers in 1 l of water makes several tens of thousands, and seaweed of Chlorella — one billion in 1 l. Dignity. effect of sewage treatment in aerobic ponds very high. Waste liquid is exempted completely from eggs of helminths, colibacillus dies off for 95,9 — 99,9%, causative agents of intestinal infections almost completely perish, size BPK decreases to 98%, relative stability increases to 99%. However vigorous oxidizing processes go in ponds only in the summer, at water temperature 8 — 10 ° they are slowed down twice, and at t ° 6 ° practically fade. Therefore biological ponds as independent treatment facilities are allowed to be applied in areas with an annual average temperature more than 10 ° and to seasonal work in a midland from May to October. For after-treatment of drain waters they can be used in all climatic areas, except for Far North where their use only in summertime is allowed.

Anaerobic ponds represent deep septic tanks to which waste liquid without primary upholding comes. Decomposition of organic matter in them is carried out by the anaerobic microorganisms (see Anaerobe bacterias) causing metane fermentation at the size pH from 8,5 and above. Alkaline condition — one of the main conditions of process of anaerobic cleaning. Duration of stay of a drain water in a pond at t ° to 12 ° — 50 days, higher than 12 ° — 30 days. Loading on BPK in days at a temperature up to 12 ° — 350 kg/hectare, higher than 12 ° — 600 kg/hectare. Decrease in BPK of a drain water during stay it in anaerobic ponds is recommended to be taken for 50%. Such ponds have a number of defects in comparison with aerobic: lower effect of cleaning of organic matters, pollution of free air badly the smelling gases of putrefactive fermentation, danger of pollution of subsoil waters pathogenic microorganisms.

Fig. 1. Aerotanks (Kharkiv biological station).

Aerotanks — flowing tanks (fig. 1) with artificial aeration of the drain waters which are previously clarified in a settler in the presence of active silt. Aeration of liquid is reached by various aerators: pneumatic, mechanical and mixed. Active silt presents itself a biocenosis of microorganisms mineralizers in the form of the flakes capable to occlude on the surface and to oxidize organic matters of waste liquid in the presence of oxygen of air. The fullest researches on clarification of a role and interaction of the major factors defining the course of oxidizing processes in aerotanks are executed by the Soviet scientific G. N. Stroganov, K. N. Korolkov, N. A. Bazyakina. They offered methods of calculation of aerotanks which use in domestic practice.

Fig. 2. Aerotanks (plan): 1 — inflow of drain waters; 2 — an input of aerated water in secondary settlers; 3 — filtrososa; 4 — air delivery from the compressor; 5 — the regulator of flow of water (shooters specified the direction of the movement of water, by an arrow on the right below — the movement of air).

Aerotanks are designed from 3 to 6 m in depth, about 8 m wide and length several tens of meters. For compactness instead of a direct long corridor parallel pieces with the barriers which are not reaching an opposite short wall are created, and water passes the necessary way, doing several turns (fig. 2). Aeration not only oxygenates water, but also does not allow to settle to flakes of active silt, bringing all mass of water into contact with them.

The B. lake of a drain water in the aerotank goes to 4 phases. The I phase — biosorption of organic matters flakes of active silt. The II phase — biochemical oxidation of easily oxidized carbon-containing organic matters of drain waters to carbon dioxide gas and water. In the same phase the energy used by microorganisms for synthesis of cellular substance of active silt is emitted. The III phase — synthesis of cellular substance of active silt from the remained organic matters of drain waters due to the energy released in the second phase.

Synthesis of cellular substance is followed by increase in content of cellular nitrogen. The IV phase — oxidation of cellular substance of active silt. Then there comes the nitrification of the ammonium salts released from the oxidized active silt. The nitrites formed at the same time partially are exposed to a denitrification with involvement of products of a denitrification for synthesis of new cellular substance or with release of free nitrogen in the atmosphere.

Knowledge of phases of cleaning allowed to offer a number of kinds of aerotanks in which one of the listed phases prevails. So, use only of the first phase of cleaning led to creation of aerotanks on incomplete cleaning and aerotanks with contact and stabilization process.

Use of the first three phases led to creation of various designs of aerotanks on full cleaning. At last, use of all four phases led to creation of installations with full oxidation of drain waters. The scheme of operation of the aerotank on full B. the lake can be presented so. The clarified drain waters come to the long concrete tank filled with mix of drain waters and active silt and, moving, are aerated within 5 — 7 hours. After the end of processes of biochemical oxidation of organic matters mix of active silt and drain waters is forced out by new portions of drain waters and comes to secondary settlers for department of active silt. The water clarified in settlers after disinfecting usually is dumped in a reservoir, and active silt goes to the aerotank again. As a result of such recirculation, and also reproduction of aerobic microorganisms the mass of active silt all the time increases and therefore excess of active silt should be deleted on silt platforms or in methane-tanks for disinfecting.

Sometimes on the way between a secondary settler and the aerotank put one more construction called by a regenerator. The regenerator is an aerotank through which there passes only active silt which is intensively blown by air. Purpose of a regenerator consists in a dookisleniya of the organic matters adsorbed by active silt and recovery of ability to participation in a refining process. It is especially important when aerotanks work for incomplete cleaning.

B.'s effect of the lake of drain waters in the aerotanks working for incomplete cleaning makes 50% on BPK, and on full cleaning final BPK of the purified waste liquid shall make no more than 15 mg/l. The maximum technical capabilities of modern aerotanks on full oxidation are that that it is possible to receive the purified waste liquid with BPK of 4 — 6 mg/l. In the course of B. of the lake in aerotanks waste liquid is released for 95 — 99% of causative agents of intestinal infections, enteroviruses, eggs of helminths. However even at high (to 99%) effect of cleaning the absolute quantity of sanitary and indicative and pathogenic microorganisms can reach several thousand in 1 ml. Therefore the sewage purified in aerotanks is always dangerous in the epidemiological relation and demands disinfecting before release in a reservoir, and the active silt occluding on itself pathogenic microflora — disinfectings on silt platforms or in methane-tanks.

See also Bacteria beds , Drain waters .

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E. I. Goncharuk.