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Small sewage treatment plants (MSW)

Distinctive features. Problems of design and operation.
PLATON technology.

Treatment of small volumes of wastewater is becoming increasingly important. Today, the construction of relatively small (compared to the scale of buildings of the 70s of the last century) housing complexes, cottage townships, restaurant and hotel complexes, etc. is very common. These objects are usually located outside the urban area and, as as a result, they are removed from the existing city sewerage networks at a distance sufficient to form the terms of reference for the construction of local treatment facilities.

This article will focus on small wastewater treatment plants (hereinafter STP) with a capacity of 10 to 1000 m3/day.

In Europe, MWWTPs are quite widespread, while the treatment indicators for small volumes of wastewater are significantly lower than in Ukraine. Just look at the comparative table of permissible concentrations of the main contaminants contained in treated wastewater.





Допустимые концентрации основных загрязнений, содержащихся в очищенной сточной воде.

Требования ЕЭС*


Украинские требования             для водных объектов культурно-бытового назначения

Биологическое потребление кислорода (БПК5), мг О2

30 – 60


Химическое потребление кислорода (ХПК), мг О2



                                              Взвешенные вещества, мг/л

35 – 60


Азот аммонийных солей, мг/л как правило, не нормируется   ( в реальности 8 – 12 мг/л)


Нитраты, мг/л

как правило, не нормируется   ( в реальности 60 – 80 мг/л)


Фосфаты, мг/л как правило, не нормируется   ( в реальности 12 – 16 мг/л)


*The data given in column 2 are valid for wastewater treatment plants that receive wastewater from facilities with an equivalent number of inhabitants from 2,000 to 10,000 people, as well as for those cases when treated wastewater is discharged into water bodies not subject to eutrophication.

**Resolution of the Cabinet of Ministers of Ukraine dated 25.03. 1999 No. 465 “On the confirmation of the rules for the protection of surface waters from the brooding by return waters” p.19

*** The values ​​are approximate, since the MPC of these indicators is regulated by local authorities authorized to issue permits for special water use. (Resolution of the Cabinet of Ministers of Ukraine dated March 25, 1999 No. 465 p. 19)

As can be seen from the table, the requirements for wastewater treatment in Ukraine have remained high since the times of the USSR, regardless of the volume of wastewater treated. To this it should be added that the cost of treatment facilities is in quadratic dependence on the effect of treatment. In other words, the price of an MOC with an 85% cleaning effect will be 2-2.5 times less than a MOC with a 95% effect. In today’s conditions, this state of affairs rather forces us to look for “other ways” of waste disposal.

No less depressing is the situation with the existing treatment facilities. There are not even official statistics on the discharge of untreated wastewater from small settlements. Having the experience of numerous trips to small towns and villages in Ukraine, I can safely assume that, according to the most favorable estimate, more than 80% of wastewater is discharged either insufficiently treated, or without any treatment at all. Under “insufficiently treated wastewater” it should be understood that wastewater passes through treatment facilities in transit. At best, only sedimentation tanks work. It should also be noted that the smaller the settlement, the greater the likelihood of untreated wastewater discharge.

In part, this can be explained by the fact that the last 25 years can be called a period of destruction of public utilities infrastructure. The most unlucky sewage treatment plants. If it is very difficult to live without water and electricity, then without treatment facilities, if you wince a little, then you can. If only the sewer was more authentic and worked properly. Everything that was once created is in an emergency state and continues to collapse. There are many reasons for this situation, including the following:

– in the pursuit of versatility and speed, so characteristic of a socialist planned economy, sewage treatment plants were often built of poor quality and from poor quality materials (ferrous metal, “low-cement” concrete, fragile asbestos-cement or cast-iron pipes);

– the smaller the settlement, the greater the unit cost of treating 1 m3 of wastewater.

– Due to the constant curbing of the growth of utility tariffs and the meagerness of the local budget of small towns and villages, it is very difficult and costly to operate such a difficult facility as a wastewater treatment plant built according to a standard project of the 70s.

As a result, the following picture is observed: disconnected blowers and sludge pumps (often without electric motors, which have found other uses), crumbling concrete of sedimentation tanks and aeration tanks, rotten sludge pipes and aeration systems, an unpleasant smell (since the structures have turned into a chain of ordinary septic tanks). Service stopped altogether, staff reduced. Sometimes you can meet the duty “operator”-watchman.

“Understanding the situation”, the controlling services simply turn a blind eye.

If yesterday the overhaul and reconstruction of treatment facilities were considered an urgent problem, today, in most cases, it will be cheaper to build new ones.

The technology for treating small volumes of wastewater in the Soviet Union developed poorly, since scientific and design institutes worked mainly on large wastewater treatment plants, small wastewater treatment plants were rather rare. Until recently, the main solution for the disposal of small volumes of wastewater was a septic tank with drainage. The geometric reduction of large wastewater treatment plants turned out to be an unacceptable solution. To create MOCs that provide the required high purification rates, a fundamentally new approach is needed.

Today, as demand creates supply, the market offers a rich palette of technological and design solutions for wastewater treatment and disposal. Equally rich is the variety of quality of the resulting product (treated wastewater).

When choosing a treatment plant, the customer takes into account the following factors:

– price;

– guarantee period;

– service cost;

– operating costs (electricity, wages, etc.).

At the same time, it is necessary for the customer to form an understanding that the quality and stability of wastewater treatment indicators are of paramount importance, and this cannot be achieved in primitive MWWTPs.

The main problems and distinctive features of the treatment of small volumes of wastewater:

1. Small wastewater treatment plants receive fresh concentrated waste, in which the amount of organic matter, nitrogen and phosphorus does not correspond to the optimal ratio for the biological process – 100:5:1 (organic matter: nitrogen: phosphorus);

2. Possibility of volley inflow of sewage. Sometimes, up to 25% of the daily inflow can reach the facilities in a few minutes. The installation must accept a burst discharge without the removal of activated sludge with treated wastewater;

3. Possibility of long-term absence of sewage inflow to the plant;

4. Possibility of salvo dumping of a large amount of synthetic surface-active substances (surfactants) formed during washing;

5. The possibility of volley discharge of highly concentrated wastewater to the plant, for example, from the kitchen, while the BOD of incoming wastewater at the MWWTP can reach up to 2000 mg/l (5 times higher than the norm and even more);

6. Minimum participation of service personnel. The cleaning process should be carried out automatically.

This is an incomplete list of problematic issues that need to be addressed when creating a technology for treating small volumes of wastewater. Large wastewater treatment plants do not have such problems, since they receive a more or less uniform flow, averaged both by the flow of incoming wastewater and by the concentrations of contaminants contained in them. Averaging here occurs in sewer networks during the movement of effluents to treatment facilities. Also, municipal wastewater is diluted with almost pure storm water, and industrial wastewater, in which, as a rule, unlike domestic wastewater, nitrogen and phosphorus are present in small concentrations.

No urban wastewater treatment plant is able to cope with such a concentration of surfactants, fats and disinfectant solutions that come with wastewater to small wastewater treatment plants, for example, from a cottage when washing clothes, cooking or washing plumbing and floors.

The above difficult conditions for wastewater treatment at small treatment facilities make it unacceptable to design them by analogy with large ones, especially by their geometric reduction.

Taking into account the above differences between MWWTP and urban wastewater treatment plants, and above all, the lack of permanently present personnel, small facilities, as a rule, are devoid of bulky mechanical treatment units and sand traps. Often fur lattices. purifications replace mesh containers, and sand traps (due to small volumes of incoming sand) are completely absent. Many MWWTPs do not have primary settling tanks, which simplifies operation, but increases the load on the aerotanks (which many people forget when calculating biological treatment units).

The traditional scheme for the treatment of domestic wastewater.

For further understanding of the design features of the MWWTP, it is necessary to present the classical (traditional) technology of domestic wastewater treatment. There are several (in fact, very few) technological schemes for the treatment of domestic wastewater. The most common is the following:

1. Mechanical cleaning of solid waste on grates, rakes, etc.

2. Removal of sand in sand traps.

3. Effluent clarification by settling in primary clarifiers.

4. Biological treatment in the presence of free-floating activated sludge and dissolved oxygen in aeration tanks.

5. Separation of treated waste water from activated sludge in secondary clarifiers.

6. Disinfection with active chlorine with a 30-minute contact in contact tanks.

Often, especially at treatment facilities of small capacity, gravel biofilters can be found instead of aerotanks. Here, activated sludge is a biofilm placed on the surface of a gravel bed. Such structures, as a rule, are subject to irreversible pollution, and already in the 2nd year of life, the cleaning efficiency drops sharply until secondary pollution appears. The resumption of the work of such biofilters is a very laborious task. The overhaul of such structures becomes economically inexpedient.

Biological treatment remains common to all facilities, both small and large. In most cases, aerobic treatment with activated sludge (in the presence of dissolved oxygen) is used to treat domestic wastewater. Structures in which aerobic biological treatment is carried out are called aerotanks. Recently, for a more complete removal of nitrates and phosphates, biological treatment facilities have become widespread, in which there are both aerobic zones, where air is supplied, and anaerobic zones, where air is not supplied. Such structures are more correctly called bioreactors. But both in aerotanks and in bioreactors, activated sludge is the main treatment agent.

Activated sludge is a biocenosis of accumulations (colonies) of bacteria and protozoa that are involved in wastewater treatment.

Biological wastewater treatment is carried out in order to remove organic substances from them, including nitrogen and phosphorus compounds.

Biological purification process.

The biological treatment method is based on the ability of activated sludge to use pollutants as its nutrition under certain conditions. Many microorganisms that make up the activated sludge of the bioreactor, being in the waste liquid, absorb pollutants into the cell, where they undergo biochemical transformations under the influence of enzymes. Often, the pre-treatment of contaminants with enzymes is done outside the cell. At the same time, organic and some types of inorganic pollutants are used by the bacterial cell in two directions:

1. Biological oxidation in the presence of oxygen to harmless products (carbon dioxide and water):

Organic matter + O2 (in the presence of enzymes) ⇒ CO2 + H2O + Q

where Q is the released energy used by the cell to ensure its life activity (movement, respiration, reproduction, etc.).

2. Synthesis of a new cell (reproduction):

Organic matter + N + P + Q (in the presence of enzymes) ⇒ NEW CELL

The intensity and depth of the processes depends on the qualitative composition of the activated sludge, the variety of forms and types of microorganisms, the ability of their adaptation (adaptation) to the specific composition of pollutants and the conditions of the process.

Process conditions.

– compliance with maximum permissible concentrations of pollutants;

– the absence of substances toxic to microorganisms in the waste liquid;

– sufficient amount of oxygen and intensity of aeration;

– the temperature regime must be within certain limits;

– the load on the sludge in terms of the amount of pollutants must be within certain limits (between the minimum and maximum allowable values);

– the contact time of the sludge and the waste liquid must not be less than the calculated period;

– the ratio of organic carbon, biogenic elements (nitrogen and phosphorus) and microelements (copper, iron, sulfur, etc.) must be within certain limits (between the minimum and maximum allowable values);

Monitoring the status of activated sludge.

Microorganisms are an effective indicator for determining the quality of the sludge. To exercise control, a hydrobiological analysis of the water-silt mixture is carried out by microscopy (study of the silt biocenosis under a microscope). The structural features of the biocenosis of activated sludge are determined, the organisms of which have the ability to respond (by qualitative change and quantitative distribution of individual groups) to the composition and properties of treated wastewater, as well as to life support conditions. The numerical predominance of one or another component of the biocenosis serves as an indicator of the stability and efficiency of the wastewater treatment process. This method allows you to determine the deviations of microorganisms and the change in the species composition of the biocenosis from the normal state. Moreover, according to the degree of such deviations, it is possible not only to determine the state of the biological treatment process and the reasons for its deviations from the norm, but also to predict the timing and prospects for changing its course.


Cleaning automation is often considered redundant. Those who argue that automation of wastewater treatment processes is not needed, that automated plants are less reliable than simple plants, have a vague idea of ​​​​biological wastewater treatment. In fact, the concept of MTP reliability means the stability of ongoing biological treatment processes, which are a necessary condition for the efficient operation of the OS and the required indicators of wastewater treatment. The system must stably keep all the parameters of the biological process within the required limits, automatically switching to the required operating mode at the right time. And yet, it should be quite simple and understandable for the staff.

Some “cheap” and “reliable” MOCs on the Ukrainian market.

As a rule, the big “secret” of their technologies is made, as a rule, by firms producing the most primitive MOS. They understand that having described their technology in detail on the website and prospectuses, it will become clear to many that such a system will not work. Small treatment facilities with a volume not exceeding the daily inflow of effluents operate as flow-through ones, which leads to the release of activated sludge from the settling tank when a volley of wastewater enters the plant. The required upflow velocity for effective settling in the secondary clarifier should be calculated in fractions of a millimeter per second. And in such installations, for example – with a volley of sewage in the amount of 0.1 m3 / min (emptying the bath), for an installation with a capacity of 1.5 m3 / day, the upward flow rate in the secondary sump will be more than 10 mm / second, which will lead to the removal of activated sludge from the installation with subsequent failure of the drainage system. It is unauthorized emissions from the installation of excess activated sludge with treated wastewater that in such systems “solve the issue” of removing excess activated sludge. After that, of course, it can be argued, ignoring the well-known laws of nature, that excess sludge does not form in such installations, or that it is enough to remove it once a year. In such installations, with an increase in the amount of incoming wastewater, the processing time decreases, which naturally affects the treatment effect. This is one of the many problems of such “cheap, simple and reliable” wastewater treatment systems. To this we can add the fact that such installations are not so cheap, and if we add to this the excessive consumption of electricity during the period when there is no sewage flow to the installation and clogging (clogging) of the drainage site, then this cheapness may turn out to be “golden”.

When considering the possibility of using a particular system from a legal point of view, the manufacturers of many MOS present a Sanitary and Epidemiological Conclusion. However, if you read it carefully, it turns out that this conclusion is given to the fact that the installation itself meets sanitary standards, but not the water it purifies. And this is naturally correct, since under the current legislation, a permit for the operation of treatment facilities can only be issued after they are put into operation, and environmental authorities will analyze the operation of the treatment facilities, take samples of treated water, conduct appropriate analyzes and issue a conclusion on compliance of these treatment facilities with the required standards. Naturally, you must first obtain approval for the project in all relevant instances. In fact, the installation is one, and the conditions for its operation are different everywhere. This permit is not required if the owner will be discharging reclaimed water into a drainage ditch on his property. A discharge permit is required only if the owner of the treatment plant decides to discharge effluent into surface water bodies for various purposes or onto terrain, within a settlement or in reaction zones. But in this case, it is necessary to obtain permission from other local authorities (administration), and most importantly, neighbors in the area.

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