Choosing the local sewage treatment plant (LSTP), the client takes into consideration the price, the warranty period, the service costs, the consumption of electricity and so on, but even if one wants to simplify and to cheapen the solving of sewage treatment problem, one should understand that the quality and the stability of the necessary parameters of the cleared sewage are of the greatest importance, and primitive LSTP simply cannot provide it.
A couple of words about LSTP
The European countries have been working on LSTP for a long time already, but the norms of quality for clearing of small volumes of sewage are very low (the biochemical oxygen consumption is 30 - 60 mg/l, the weigh is 30 - 60 mg/l, nitrogen and phosphorous are not normed at all as a rule), so there was no reason for developers in Europe to work at effective small clearing constructions with such low requirements to cleared sewage. As far as the city sewage treatment plants are concerned, one could learn much from western developers, as the requirements of the cleared sewage get higher and more serious with the growth of a sewage treatment plant (STP) productivity. The small clearing constructions were also developed on the pattern of city STP, only scaled, but this approach is not acceptable.
As a result of the above-stated reasons there was a certain vacuum in the field of small clearing constructions which gave a push to creation of new technology of biological sewage treatment BIOTAL.
What are the main problems of treatment of small volumes of sewage?
- The fresh concentrated sewage goes into the small clearing constructions, and the quantity of organic substances, nitrogen and phosphorus in such sewage sometimes exceeds the normal for biological process corellation - 100:5:1 (organic : nitrogen : phosphorus), that is why in primitive unautomized systems these pollutants, that exceed the corellation, will flow away from the station together with the cleared sewage.
- The volley inflow of sewage can bring to the plant over 20% of day inflow in a few minutes. The plant should be able to accept the volley inflow without the outflow of sludge with the cleared wastewater.
- The long absence of sewage inflow, for instance in the vacation period, is also a problem. If there will be no automatic power regulation, the self-oxidation (the dying off) of the active sludge will take place.
- The pollutants, that are toxic for active sludge microorganisms may appear in the sewage. For example the inflow of a great quantity of detergents during the laundry.
- The sewage with high concentration of basic pollutants can inflow into the plant, for example the sewage from kitchen. In this case the BOC of the sewage may reach up to 2000 mg/l, and it is known that for BOC more than 500 mg/l one must foresee at least double-sludge system with multiplanimetric recirculation of returnable active sludge.
- The growing of the active sludge quantity during the process of treatment. If it is not removed automatically, it will flow away with the cleared sewage after reaching the critical concentration
- The absence of the personnel. The process of treatment should pass in automatic mode.
This is an incomplete list of the problems that should be solved in the technology of sewage treatment. Large STP do not meet such problems, because the inflowing wastewater is more or less similar from day to day, and it is already cleared by 20% in the sewarage network, and it is also diluted with the clean waters, mixed with industrial sewage, that usually has a shortage of phosphorous. As a result this sewage comes to treatment plants as a "cocktail" perfect for mircoorganisms. Small sewage treatment systems do not meet the inflowing waste-water like that.
Experts in the field of sewage treatment know that small clearing construction should be designed at higher technological level than big ones because it should provide required quality of sewage treatment in extreme conditions without the constant attendants and with the minimal expenses for their exploitation (electric and thermal energy, etc.).
For example the small clearing constructions treating sewage from a cottage should be solved at higher technological level than the big clearing constructions of city of Paris. A favourite phrase of some designers of small clearing constructions «... we use the operational experience of the big clearing constructions during the development of our technology of small construction…» is absurd as the listed difficult conditions of sewage treatment on small clearing constructions, make unacceptable their designing by analogy of big construction especially by the way of their geometrical reduction.
Let us review some technological aspects of the biological sewage treatment, that one needs to know to understand the processes that take place in LSTP. The plant of biological sewage treatment represents «an alive organism», consisting of millions "workers - bacteria" eating pollutants and clearing sewage.
Continuous and discontinuous ways of sewage treatment
There are two ways of processing the sewage - a continuous one, when the sewage is processed moving from one zone of plant to another, and a discontinuous one (an SBR-reactor), when all the cycles of purifying take place in one container, only conditions - aerating, mixing, sedimentation, surplus sludge removal - follow one another. Both ways have their own advantages and disadvantages.
The continuous way of sewage treatment doesn't permit to support constant active sludge concentration in the plant within 5 - 6 g/l, which is necessary for oxidation of the increased quantity of fats and detergents, that came with the sewage into the plant, because during the volley sewage inflow it can be taken away with the ascending waterflow in the sedimentation reservoir. A serious disadvantage of the continuous way of clearing sewage is also the sedimentation and rotting of the active sludge in the secondary sedimentation container. There is no rhythmic interchange of oxidation and restoration processes. In the period of minimal or maximum inflow the time of sewage processing is not correct. The sticking of sludge on the walls of the secondary sedimentation reservoir and its coming to the surface as a result of unallowed denitrification with the outflow together with the cleared sewage is also a problem of such systems. A serious problem for such systems is also the necessity of removal of the surfacial pollutants (fats, the particles of active sludge and so on) from the sedimentary containers.
The discontinuous way of sewage treatment (SBR-reactor) hasn't got the listed problems, but it has its disadvantages. The active sludge, adapted to certain conditions of the sewage requires a significant period of time for adaptation to new wastewater portion, the rate of purification in this period is low. After it is partly adapted to new conditions, another cycle starts and the problems repeat. One of the main laws of engineering chemistry is not kept here - the process should last as long as it is possible. The SBR-reactors are made for 4-hour purification cycle, and only light organic pollutants are oxydized during it, the water purification is insufficient. The nitrification process takes place only when almost all the organic substances are oxydized, so reaching denitrification (the necessary conditions of which are complete nitrification and the availability of light organic substances) is impossible in SBR-system, because it is closed and the light organic is already oxydized. However this sytem has also got a number of advantages. It allows to support the high concentration of the active sludge in the system without the risk that it will outflow with the cleared water, because the sedimentation in SBR-systems takes place without any water movement. The next important advantage is that there is no necessity of removing the surfacial pollutants, because the cleared sewage is pumped from the clarified layer of water, under the level of water in the container. This way of sewage processing allows to avoid building the secondary sedimentation container, as the aerotank can be a sedimentation container itself when the aeration is off.
Therefore, because of the complication of sewage treatment of small volumes, the LSTP should have advantages of both ways of sewage processing, and it should have no their disadvantages.
Recirculation of returnable active sludge
What is the recirculation of returnable active sludge needed for? It is one of the main conditions for a good work of sewage treatment system. In the beginning of the system the sludge absorbs the organic pollutants, and then it oxydizes them moving from one clearing zone to another, and gets regenerated and hungry, having eaten all the absorbed pollutants. The returnable active sludge, being hungry, gets into the accepting container and more effectively removes the pollution from fresh sewage. If one doesn't bring the returnable active sludge back to the beginning of the plant, the sludge in the end of the system will mineralize itself (simply die, because there are no organic substances for its nutrition), and at the beginning of the system the sludge will be overloaded and will not work effectively. The denitrification - the process of nitrogen removal - will not take place if there is no recirculation of returnable active sludge, because denitrification requires the availability of light organic substances. The gaseous nitrogen after denitrification goes back to atmosphere. As the ammonia nitrogen is oxydized to nitrites and nitrates only after the most of organic substances are removed from the sewage (it is the peculiarity of the biological process), that is to say in the last and in the next to last reactors, the only chance to make fresh organics and nitrates meet is to provide the recirculation of returnable active sludge. It is not the complete list of the advantages of recirculation, the dissolving of toxic pollutants also takes place thanks to it.
The extended aeration with cyclic processes of aeration and mixing in reactors
During the extended aeration with cyclic processes of aeration and mixing combined with the more than 25 days age of active sludge, the facultative microorganisms appear, which take an active part in the purification processes both in aerobic and unoxyde conditions. Thanks to it, the quantity of aerobic active sludge in the system increases, the nitrification and denitrification bacteria appear - as a result the nitrogen and partly phosphorous are removed from the sewage in biological way.
The removal of the excessive active sludge
During the extended aeration the active sludge grows in quantity 0,25 - 0,35 mg of BOC taken, and during the sewage treatment on the city plants it grows up to 0,8 mg. The surplus active sludge should be removed from the system regularly. Some constructors' statement that the surplus active sludge practically is not formed in their systems and that it can be removed twice a year is absurd, and it is almost the same as the statement that someone has three meals a day, but uses a toilet twice a year. A man is also like a bacteria, only a huge one, the biological processes during the digestion are like the processes that take place during the oxydation of the pollutants by the bacteria of active sludge. However the "surplus active sludge" of the human being is a "food" for the microorganisms of the active sludge in the plant. In other words, the bacteria of active sludge oxydize what a man hasn't.
As it was said above, concentration of active sludge in the system should be within the limits of 5-6 g/l for effective sewage treatment. At the greater concentration of active sludge there will be a secondary pollution of cleared sewage, and at smaller concentration of active sludge the system will not cope with clearing volley (organic) inflows of sewage. No city sewage treatment plant is capable to cope with such concentration of detergent, fats and disinfectant solutions which inflow with sewage on small clearing constructions (for example from a cottage, during the laundry, preparation of meals or washing sanitary engineering devices and floors).
The surplus active sludge dewatering
The dewatering of surplus sludge is made for decreasing the volume of a main product of sewage purification, for cheapening and simplifying of operations with it and its transportation to the utilization place. The variety of devices exists for dewatering the surplus active sludge, but most of them are expensive and require the use of the floculants, the process is accompanied by an unpleasant smell and the personnel for it is also needed. But one can choose another way for the plants up to 1 000 cubic metres per day. Since the active sludge is of a significant age, and thanks to extended aeration (for more than 25 days), and therefore thanks to the high mineralization of the active sludge, its stabilization in the aerobic stabilization container is sufficient. After that its mineralization is almost complete, and its dewatering can take place in the sludge-bags without adding any floculants. All the process of dewatering in this case is not very complicated and can be easily automatized.
The surplus active sludge growth
As it was mentioned before, during the extended aeration the active sludge grows in quantity 0,25 - 0,35 mg of BOC taken, If by some reasons the active sludge in the plant gets "sick", becomes opressed, the bacteria practically stop oxydating the pollutants and start to sorb them, the active sludge volume grows significantly, which leads to the infringement of the purification process. There can be a number of reasons for that - the inflow of more sewage and organic pollutants than it was predicted by the project of a plant (the active sludge does not cope with the pollutants); the inflow of fats and detergents in the quantities that exceed the allowed concentration (in this case the flakes of sludge are covered with a thin layer of these pollutants, that prevent oxygen from getting inside such flake); the inflow of the sewage that contains toxic substances in quantities higher than it is allowed for accepting it into the sewarage systems, the sewage temperature less than 5 degrees, the pH out of limits 6,5 - 8,5 and so on. One shouldn't also arrange any collecting containers without aeration, because an anaerobic process with hydrogen sulphide exude will take place there, and it will depress the active sludge bacteria in the plant. The local STP can only process the sewage parameters of which allow its accepting into the city sewage treatment plants, in other case one should also foresee the precleaning.
The biological way of removal of nitrogen and phosphorous
One of the basic polluting elements in sewage is nitrogen and phosphorous, and creating the conditions for their simultaneous removal from the sewage is necessary. One should provide the alternation of aerobic and anaerobic conditions in the zones of the plant together with more thant 25 days age of active sludge for that. It is necessary to foresee the two-stage nitrification and denitrification, as these processes are very complicated, and the concentrations of the ammonia nitrogen, nitrites and nitrates and light organic substances are changed abruptly all the time. For example, if there is a significant quantity of ammonia nitrogen, it will be oxydized to nitrites and nitrates; but of there are no light organic substances the denitrification won't pass and the necessary parameters of nitrogen won't be provided at the outflow. And when the plant has got a few zones of purification with multiplanimetric recirculation of the active sludge, the nitrification will pass, as it starts after main part of organic substances if oxydized, which can't happen in one aerotank, and nitrites and nitrates will meet the light organic substances sooner or later in the conditions of oxygen lack for denitrification. The phosphorous removal passes mainly because the surplus active sludge removal, where it is absorbed by PP-bacteria.The common active sludge contains 1,5 - 2% of phosphorous, and the sludge, that is exposed to alternation of aerobic and anoxyde conditions, contains up to 6 - 8% of it. The surplus active sludge should be removed automatically from the aerobic zone, because the phosphorous, accumulated by the PP-bacteria in it dissolves after getting into anoxyde conditions.
The automatization of the biological sewage treatment processes on the LSTP
The booklets of some manufacturers call the automatization of the sewage treatment just a "fashion". Those, who state that the processes of sewage treatment on LSTP doesn't require automatization, that automatized plants are less reliable, than unautomatized ones (the plants that work like septic tanks) have a gloomy idea of biological sewage treatment. In fact one should understand under the reliability of LSTP the stability of running processes, which provide the necessary high parameters of cleared sewage. In another case the sewage will "effectively" flow through all the plant during the volley inflow, this will destroy the drainage system. This will be the price of the "simple and cheap" clearing construction. We usually like simple and cheap decisions - learn english during one night, become rich with making no efforts and so on, this is deep in our idle nature, but as it is known it never results in something really good. An example of automatization can be a modern washing machine, a drink water purification system and so on. The home pool, that has automatic system of defining the chlorine quantity and pH, automatically doses the reagents and then washes the filter, where the man does but one thing - bathing. There is another variant however, no automatization, the drifting chloring tablette, control papers for defining pH, washing the filter by hand, setting the temperature - constant care of a construction, and if you still have time after this - you'll bathe there. So, the "simplicity" in the modern world is an argument for old ladies with the humanitarian education. However, each complicated decision should be justified, and it must be made for reaching the main purpose, in our case it's the needed effectivity of purification, the reliability of work, electricity economy and the exploitation of plants (even up to 1000 cubic metres per day) without constant staff.
The visual defining of the biological process infringement (the dimness of cleared sewage, the "swelling" of sludge and so on) takes 2 - 3 weeks work for its regeneration, because it is a biological process. If there is no automatization, the personnel will only be able to state that the system doesn't work correctly and will try to correct it in the manual mode. And if the system is automatized, it constantly holds all the parameters in necessary limits.
The usage of new hydrodevices and processes in LSTP
To create a guided and self-regulated hydro-pneumo-biological system, one needs to create and use new self-regulated hydrodevices because of the complication of the biological process, the irregularity of sewage inflow, the inflow of substances toxic for active sludge microorgainsms (detergents, desinfecting solutions, manganese salts and so on). These devices must allow
- keeping certain levels of water in different plant zones to create accumulating volumes for accepting volley inflows of sewage;
- providing the recirculation of the sludge between the reactors in proportion to the quantity of accepted sewage;
- pumping out of the cleared sewage after sedimentation and guaranteeing the abscence of the floating pollutants and active sludge particles in the outflowing water;
- providing the uniting of a number of biological processes inside one construction.
So, now we can state what basic requirements should be met by a LSTP (1,5 - 1000 m3/day), so that it would be able to provide effective sewage treatment with no personnel in conditions of constantly growing prices of energy sources.
LSTP should be constructed with such principles (and by these criteria should be estimated):
- They must have advantages of the continuous and discontinuous systems (reactor SBR) of clearing but mustn't have their disadvantages;
- They should detain the rough rubbish and make it small;
- The rough rubbish detaining system in the accepting container should have a system of self-cleaning;
- They must cope with the volley inflow of sewage not less than 25% of the daily charge without carrying out of sludge from the plant;
- The system must be multistage and it should have the multiplanimetric recirculation of returnable active sludge;
-The technology should have at least a double-sludge system;
-The hydraulic system of LSTP should provide the levelling of volley inflows of the sewage and multiplanimetric recirculation of returnable active sludge and the degree of recirculation should change proportionally to quantity of sewage arriving into the plant;
- To provide the nitrogen removal in the biological way, creating the conditions for passing the double-stage process of nitrification and denitrification;
- To remove the active sludge automatically;
-To support required concentration of active sludge in system automatically with the possibility of its correction;
- To have automatized system of aerobic active sludge stabilization and its dewatering without floculants;
- To adjust the capacity automatically depending on quantity of inflowing sewage and to have some levels of economical modes depending on time of absence of receipting sewage. This will help to save energy, the resource of electric devices, and to help the sludge survive in the conditions of lack of food. The plant also has to pass to the forced operating mode at the volley receipt of sewage;
- To use highly reliable level sensing elements with self-cleaning system;
- To return onto the controller monitor the basic parametres of work of the plant with the possibility of their correcting in reactors: aerating time, mixing and sedimentation time, the clear water removal, - preferably with a possibility of transferring through external networks;
- To have alarm system that would give an advance notice about the infringements of the treatment process;
- To give a possibility of repairing or replacement of any unit without stopping the work of a clearing construction.
The suggested LSTP in the market
As the clients have often been led astray with the untruthful information, they don't believe anyone at the moment and become professionals in need, studying the constructions of the LSTP and the principles of sewage treatment. The purpose of this article is to provide everyone who needs it with the information about LSTP, and let the clients make a choice themselves. As a rule, a "great secret" is made of technologies by the companies that produce primitive STP. They understand, that if they describe in details the way their plants work, everyone will see that they do not work at all and that they can't provide the necessary parametres of cleared sewage.
The small STP that are available at the market, can be divided into two groups.
The first group contains the systems, that meet main requirements put to the LSTP - they work in the mode of extended aeration, the control over the process is made with the help of the controller and the process itself is divided into fazes (which is a perfect decision for small STP), have accumulating volumes to accept the volley inflow of the sewage, the system of automatic surplus sludge removal, having the alarm system and so on.
The plants that belong to the second group are not able to solve main technological tasks.
The LSTP of second group as a rule have the same level of water in all the zones, that is to say they are linked hydraulically, they work as flowing, which leads to the outflow of the active sludge from the sedimentary container during the volley inflow of the sewage. The necessary speed of the upflow in the sedimentation tank should be less than a milimetre per second. And in these plants, for example when the volley inflow happens and 0,2 m3 (a bath) comes into the plant of 1,5 m3 per day, the speed of the upflow in the sedimentary container will be above 10 mm/s, which will lead to the outflow of the active sludge from the plant, which destroys the drainage system. Actually this is the way for such stations to solve the question of the surplus active sludge removal. After that one can state that the surplus active sludge in these plants almost doesn't appear and that it is quite enough to remove it twice a year. Nature is smarter than we are, and if this had been possible the men would have been created without the necessity of using the toilet. These plants work without the automatization, they do not regulate the power according to the quantity of inflowing sewage, which leads to the aforesaid problems. When the quantity of sewage in such plants changes, the time for processing it in separate zones changes too. The worst fact here is that when more sewage comes it requires more time for its processing, and in these plants it is treated less time, than necessary. These are some of many problems that are characteristic for such "cheap, simple and reliable" sewage treatment systems. One can add, that these plants are not much more expensive than the plants from the first group, and if you take into consideration the use of surplus energy when no sewage comes to the plant, this "cheapness" gets "golden".
The BIOTAL technology
No methodes that destroy the water structure or change its biological activity may be used for waste-water treatment. The parameters of cleared sewage should be close to the natural water. Actually this was the purpose of creating the BIOTAL technology, because after such treatment the water can be used again, which helps one to save drink water, the cost of which is more and more expensive each day because of the pollution of its sources by uncleared sewage. Developing the technology the disadvantages of both continuous and discontinuous systems were taken into consideration and avoided, and the advantages were used. As the technology was made "from zero", and no other technology was taken as base, so it hasn't got the defects of other technologies. Certainly, it had its own defects, but they were removed as technology developed. This lead to the creation of a series of new devices: siphon airlift (patented), the regulated siphon (patented), the regulated airlift (patented), the reverse airlift (patented), and also new technologic constructions - the accepting container - denitrificator (ACD, patented), 3-stage reactor SBR (patented), biofilter - thin-layered sedimentary (BFTL, patented). The international patent claims were made for these devices.
These devices and constructions belong to over-reliable, as there is no element to get out of order - no moving elements, and regulating these devices is made by the magnetic valves, that deliver air there according to one of 6 modes, to which the plant si switched automatically by the MITSUBISHI controller depending on the quantity of sewage. The magnetic valves ASCO (Netherlands) have a tremendous resource, millions of cycles of switching on, applying to BIOTAL technology this means 30 - 50 years of plant's work. The automatics of the plant is made of modules of the leading world's manufacturers - Mitsubishi, Moeller and so on. Even the smallest BIOTAL plants are equipped with such block of automatics, which have a monitor, where the basic parametres of plant's work are shown. This block is more expensive, than those that are semi-handicraft for a special plant, but it is much more reliable, and the service group will easily make corrections if necessary replacing any module. If any compressor is out of order, the automatics will give an alarm signal, and the system will work at least for 24 hours without making the purification worse. The level sensing elements are doubled on all the BIOTAL plants. After we introduced the system of their self-cleaning, their being out of order didn't happen at all. The controller's program also takes into consideration cases when the lower sensing element is not closed, and the upper closed (which means that the lower is out of order), the system ignores the data of the lower sensing element (while the controller shows the message about the infringement), if necessary this information can be sent with the modem to the service group. As even the smallest BIOTAL plant has a possibility to be connected to the modem, it can be connected to the system "Smart house" (certainly, if the client has got this system in his disposal). However, there is no necessity to connect the cottage plants to the external networks, because the system is reliable enough and the most important elements are doubled. However as soon as you wish to do so, it will be enough to buy a cable for a controller and to switch it to the modem.
As a result, the fully automatized, 8-stage, 3-sludge, self-regulated hydro-pneumo-biological system with 4-planimetric recirculation of returnable active sludge.
The necessity of creating this technology appeared because the effective biological treatment requires the corellation of the organic substances, nitrogen and phosphorous within 100 : 5 : 1, which never happens in reality. That is why the bacteria of active sludge will leave what they haven't eaten, those pollutants that exceed these parametres will outflow with the cleared sewage. The multiplanimetric recirculation and the multistage system makes these pollutants circulate together with the active sludge and lets them meet the new sewage where the parameters differ from time to time. So the microorganisms, that eat the pollutants in the same corellation, will eat the rest with each recirculation cycle. The BIOTAL has conditions for simultaneous removal of nitrogen and phosphorous in biological way. The alternation of aerobic and anoxyde conditions, combined with the age of sludge over 25 days is created for that. The facultative microorganisms, that take an active part in purification processes, appear in such conditions. This actually provides the effective removal of nitrogen and phosphorous in biological way.
The surplus active sludge is removed from the BIOTAL aerobic zone automatically, because the phosphorous, absorbed by the PP-bacteria in aerobic zone, dissolves as soon as it meets the lack of oxygen.
The dehydrating of surplus sludge in the sludge tank of the plant BIOTAL is made without addition any flocculating agent because of the significant age of active sludge (more than 25 days) and its strong mineralization, and also its long stabilization by aeration.
The plant BIOTAL automatically switches into one of 6 programs: forced mode (when the quantity of sewage is more than the project one). At absence of the sewage inflow into the plant BIOTAL it automatically passes in the first (in 1 hour), the second (in 24 hours) and in the third (in 168 hours) economical modes. It has allowed reducing considerably the charge of the electric power, to prolong service life of the equipment and to provide ability to live of microorganisms of active sludge at a long absence of sewage receipting. For instance when the sewage is absent for more than 7 days, the system passes into the third economic mode, which helps to save up to 80% of energy, compressors and valves resource.
BIOTAL plants from 10 to 1000 cubic metres per day
Let us look through the principal technological scheme of BIOTAL plants from 10 to 1000 cubic metres per day.
We make a start from the statement that a man uses 300 l/day of water. Most companies state, that one uses only 200 litres, which is insufficient. The BIOTAL plant consists of the accepting container-denitrificator (ACD) with a net made of stainless steel for rough rubbish removal; three-stage SBR-reactor, the aerated biological filter with plastic filling, which also works as a thin-layer sedimentation tank, contact reservoir and a surplus active sludge stabilizer with the dewatering system. As it is known, the perfect system of biological clearing should have 7 stages - 7 airtanks, that follow each other. In this case the microorganisms of different groups, that do not compete, work effectively, and certain groups remove specific pollutants, and the water is processed gradually. Only in BIOTAL plants the hydraulic connections between ACD and SBR-1, SBR-2 and SBR-3, SBR-3 and BFTST, BFTST and contact reservoir are periodically interrupted according to the program. The hydraulic connections are performed: between ACD and SBR-1 - by the ACD-pumps, SBR-2 and SBR-3 - by the controlled airlifts or reverse airlifts, SBR-3 and BFTST - by the controlled siphon, BFTST and contact reservoir are connected by hydraulic opening. Going from one zone to another the water gets clear stage by stage in 6 - 8 phases according to one of six programs, and the composition of phases differs for the economic mode - the pumping out of the cleared water and the surplus sludge removal do not take place. The BIOTAL system has three sludge system: in ACD, in the three-stage SBR-reactor and in the BFTST. The 4-planimetric recirculation of the returnable active sludge takes place. This construction of the system allowed to have the three-sludge system, as the pumping of the sludge mixture takes place after the sedimentation in the appropriate container. The sludges in the plant are partly mixed during the recirculation before the sedimentation cycles.
While being processed, the sewage gradually moves from the first to the last purification stage, which is provided by the temporary hydraulic connection between the reservoirs by the hydro-automatic devices.
The work of the plant has 6 - 8 phases, the quantity of which depends on the mode of the work of the plant.
The sewage comes through the grate where the rough rubbish is held. After that the sewage flows into the accepting container-denitrificator (ACD), which works as an SBR-reactor, as an accumulating space for the irregular inflow of the sewage, and as a first-stage denitrificator. It contains the self-cleaning non-rusting grates with double-side barbotage for upholding the rough rubbish, the aerating and mixing systems, the self-cleaning level sensing elements, and the pumps, that pump water into SBR-1. The fresh sewage is mixed here with returnable active sludge from the SBR-3, which contains the nitrites and nitrates. During the mixing mode the process of denitirification takes place and it has a double effect: the denitrification with the deliverance of gaseous nitrogen into the atmosphere and the oxidation of the organic pollutants of the fresh sewage with the oxygen of the nitrates and nitrites. The concentration of active sludge here is always constant due to the level of pumping of the previously cleaned sewage. The pumps, that pump the mixture into the SBR-1 after sedimentation in ACD, pump out the surplus active sludge from ACD at the same time. Changing the height of the pumping level one can regulate the necessary concentration of the active sludge in ACD.
The previously cleared sewage from ACD is pumped into the SBR-1. The SBR-1 is hydraulically connected with SBR-2 by an opening. The aeration in SBR-1 and SBR-2 is periodically interrupted according to the program, and the circulation of the mixture of sewage and active sludge is happening all the time. The second stage of denitrification takes place in SBR-1 during the mixing. As the process of nitrification takes place in the SBR-2, and the returnable active sludge from SBR-2 contains enough of the nitrites and nitrates, and there is still light organic in SBR-1. The denitrification can be more deep when the aeration elements are in the mixing mode - when less air is given for aerating. In this case the denitrification in SBR-1 will take place even when there is aeration in SBR-2, that is to say during all the process of purification. After these reactors the sewage is pumped by the controlled airlifts into SBR-3, they also remove scum in such a way that the microorganisms in SBR-3 are not exposed to the negative effect of the detergents. While the controlled airlifts are pumping the sewage into the third SBR reactor, the recirculation of the returnable active sludge is performed into the SBR-1 and ACD. The SBR-3 works as an aerotank at first, where the processes of hard organic oxidation and the second stage of nitrification take place. After switching off the aeration and the airlifts, it starts acting as a secondary sedimentation tank. The aeration, sedimentation and the pumping out of cleared sewage by the controlled siphon into biofilter-thin-layer sedimentation tank (BFTST), and also the pumping out of the surplus active sludge into the stabilization container with further (after stabilization) dewatering take place. During the aeration of SBR-3 the aeration of central part of the filling of the biofilter takes place, this results in the airlift effect in the cells of the filling , which leads to the recirculation of cleared sewage: in the cells, where air comes, the water moves upwards, and in the cells, where air doesn't come, the water flows down. The plastic filling of the biofilter is covered with a biological layer, and only the part where air reaches works for oxidation (continues to oxydate the hard organic substances and provides the third stage of nitrification), and the cells, where air doesn't reach, provide the third stage of denitrification. The cleared sewage from the third reactor comes to the lower part of BFTST, after the aeration, the sedimentation and the surplus sludge removal is stopped. The sewage cleared in previous cycle in BFTST is displaced by the sewage from SBR-3, and it is moving upwards through the cells of plastic filling, that now starts working as a thin-layer sedimentation tank. This provides the effect of weigh removal five times higher than during the classical sedimentation (literature data and our own experience). The replaced sewage flows into the contact reservoir, where their desinfection takes place. In its turn this water replaces the sewage, that was there before, and makes it outflow from the station.
As the composition and the inflow dynamics of sewage differ for each LSTP, the service group can easily correct the work of the plant and change the parametres of its work. This can also be done via modem connection.
The sewage inflows into the accepting container, which represents a large non-corrosive net, where the aeration element, that makes small the rough rubbish, is installed. There is an aeration element under this net, it provides the aeration of SBR-1 and outer aeration of the net. The double-side barbotage of the net is taking place, which helps to make the rough rubbish smaller and prevents the cluttering of the net. The water deprived of rough rubbish flows down into the SBR-1, where the returnable active sludge from the SBR-3 and SBR-2 comes. In the first reactor the water is partially cleared, being exposed to the repeated processes of aeration, mixing and sedimentation with the lack of oxygen, thanks to which the process of denitrification takes place (the nitrites and nitrates come with the returnable active sludge from SBR-2 and SBR-3, and the light organic substances that come with fresh sewage). After the SBR-1 the sewage flows into SBR-2, where the scum is removed by the reverse airlifts, which helps to avoid the influence of detergents over the microorganisms in SBR-3. Just as in SBR-1, the sludge mixture is exposed to repeated processes of aeration, mixing and sedimentation. As over 50% of organic substances is oxydated in SBR-1, the nitrification process starts in SBR-2 together with decomposition of organic substances. As the quantity of organic substances decreases, the process of nitrification starts to dominate. The partly cleared sewage from SBR-2 is pumped into the SBR-3 by the reverse airlifts.
The oxidation of hard organic substances and nitrification takes place in SBR-3. The conditions that are created for nitrification provide the oxidation of ammonia nitrogen to the nitrites (reduction-oxidation potential less than 100), which allows to have a faster and more effective denitrification in SBR-1, because the chain of reduction of the nitrites to the gaseous nitrogen is shorter comparing to nitrates. The sewage is exposed to aeration with further sedimentation in SBR-3, and the cleared sewage is removed by the controlled siphon into the biological filter. Before the cleared sewage is pumped out, the pumping of the surplus active sludge from SBR-3 into the aerobic sludge stabilization container takes place, and the pumping out of stabilized active sludge into the dewatering block. After the cycle of sedimentation in SBR-3 is over, the pumping out of the cleared water and its delivery into the biological filter takes place, where there is the water that was cleared in the previous cycle of purification. While the new portion of sewage comes to BFTST, it replaces the water that was purified here before, and as the aeration is off at this moment, the plastic filling begins to play the role not of a biological filter, but a of thin-layer sedimentation tank, which effectively holds the smallest weigh. The final result of this is the outflowing cleared sewage. In case that it is necessary to pump out the cleared sewage with a pump, it is set over the plastic filling of the biological filter, where the cleared water is accumulated.
Dr. TETERJA ALEXANDER
