Activated sludge process

The activated sludge process (also briefly activated sludge process ; engl .: activated sludge process ) is a process for biological wastewater treatment in sewage treatment plants . The mostly municipal wastewater is largely freed from organic impurities, i.e. cleaned, by the metabolic activity of aerobic chemoorganoheterotrophic microorganisms , the so-called activated sludge . The process starts after the mechanical pre-treatment of the wastewater. For municipal wastewater, this process is one of the classic intensive treatment processes. The general usability and the good cleaning effect for waste water to reduce the content of suspended solids, the chemical oxygen demand (COD), the biochemical oxygen demand (BOD ₅ ) and the nitrogen compounds (N) are advantageous . The main disadvantages are the high investment costs, the high energy requirement and the sensitivity to hydraulic overload.

Procedural bases

Activated sludge process in continuous operation

Plants using the activated sludge process, both continuously d. H. in continuous operation and discontinuous ( batch mode ; engl .: batch process ) are operated. What all variants have in common is that the bacterial mass ("activated sludge") suspended in the water takes over the biological cleaning of the wastewater. In the classic configuration with continuous operation, the activated sludge process consists of three main components: the activated sludge tank, the secondary clarifier and the return sludge conveyance. The inlet and return sludge from the secondary clarifier first enter the activation tank, in which the mixture of waste water and activated sludge is aerated. This is where the biological processes take place. From here, the sludge-water mixture flows into the secondary clarifier or sedimentation tank for sludge separation. The activated sludge is separated from the treated wastewater by sedimentation in the secondary clarifier, thickened by its own weight, drawn off at the bottom of the tank and (for the most part) returned to the activated sludge as so-called return sludge . This results in a high concentration of activated sludge in the activated sludge tank and thus a high efficiency of the process. The purified wastewater, which has largely been freed from the activated sludge, leaves the secondary clarifier at the top via an overflow weir.

The substances contained in the raw sewage serve the bacteria in the activated sludge as food, so that new bacteria constantly grow back. For this reason, a smaller part of the sludge is removed from the secondary clarifier as "excess sludge" in order to keep the sludge concentration in the aeration tank largely constant. This is disposed of as sewage sludge.

Several biological stages can be switched one behind the other to carry out the process in sewage treatment plants (aeration tank I, intermediate clarification tank, aeration tank II, secondary clarification tank). The sludge deposited in the intermediate clarifier is conveyed as return sludge into the first activated sludge tank , that of the secondary clarifier into the second activated sludge tank . Thus different biocenoses arise in the two stages. The higher polluted first stage can specialize in easily degradable substances or adsorption without degradation, while the second stage can specialize in poorly degradable substances and nitrification ( ammonium oxidation to nitrate ).

With discontinuous or batch operation, the individual phases of the aeration process (filling, aeration, separation, emptying) take place one after the other in just one basin (see figure).

The sewage-activated sludge mixture can be aerated by surface aerators, by blowing in compressed air or by gassing with pure oxygen. It is important to ensure that the ventilation is sufficiently designed, both in terms of the amount entered and in terms of the even distribution (complete mixing) of the oxygen, in order to cover the oxygen demand of the microorganisms in the entire volume of the aeration reactor that is necessary for the biological degradation processes . In addition to the biological oxidation of hydrocarbon compounds , oxygen is consumed in the oxidation of nitrogen and phosphorus compounds .

In the basin there should always oxygen levels of about 2 mg / l are not present unless a special strategy for denitrification (conversion of NO ₃ ^- to N ₂ ) is necessary. The ventilation can be controlled by oxygen probes or complex control mechanisms, taking nitrification / denitrification into account (time-pause control, redox potential, ON-LINE measurement of NH ₄ , NO ₃ ).

With compressed air ventilation, the ventilation capacity is regulated by switching the blower / compressor on and off or regulating the speed. In the case of surface aerators, the aerators are also switched on or off or the speed changed to change the O ₂ entry. The immersion depth of the rotors / gyroscopes can also be changed by changing the water level in the basin to regulate the oxygen input, but this is rarely practiced today.

Circulation tanks, square or elongated shapes can be used as tank shapes for aeration tanks. Depending on the operational requirements (e.g. separation of ventilated and non-aerated basins for denitrification ), several basins can be provided.

The systems are dimensioned according to the age of the sludge, which is the average length of time the bacterial sludge remains in the system. This ensures that there is sufficient time to also hold slow-growing bacteria such as nitrifying bacteria . The sludge age is basically not the hydraulic length of stay, as the sludge management is decoupled from the hydraulics to a certain extent due to the retention of the sludge in the secondary clarifier . The sludge age is therefore dependent on the amount of sludge in the system and the daily excess sludge accumulation due to the biomass growth. The classic measurement parameters of volume and sludge load (BOD ₅ per kg DS, day) can be derived from the sludge age.

For reasons of separation efficiency in the secondary clarifier, sludge concentrations of 3 to 5 g dry matter per liter can usually be kept in the aeration tank. For the cleaning with nitrification and denitrification that is common today, sludge age of 15 to 25 days is required at the low temperatures that are common in Central Europe.

The surface of the secondary clarifier is measured according to the expected sludge settling properties.

Mathematical description of the processes in activated sludge plants

The first biological sewage treatment plants were designed based on practical experience. When operating these systems, the target values ​​were checked by continuously recording the actual operating values. The process management in the plants could be optimized through evaluation and in some cases mathematically linked new key figures were obtained instead of the original empirical data. While these older key figures in the ATV 131 specifications for the design of biological wastewater treatment plants were almost exclusively based on empirically determined practical values, they were continuously improved through detailed analyzes of the practical values. Mathematical relationships were recognized that enable increasingly better economic efficiency for the design and operation of the systems. The original "ATV 131" evolved into the current "DWA-A 131", which is supplemented by additional worksheets and further specifications such as "ATV-M 210" and "DWA-M 210".

The processes in sewage treatment plants can be described mathematically by their reaction kinetics (see also Michaelis-Menten theory, especially for the biochemical processes) and hydraulics . This is particularly possible for the processes in the aeration tank. The complicated processes in the secondary clarifier (flocculation, thickening, settling, currents, etc.) are much more difficult to grasp mathematically.

Although unsteady models of the activated sludge process have already been developed, the dimensioning is mostly based on steady-state assumptions, in particular for the design according to the sludge age, i.e. the mean residence time of the activated sludge. This is to ensure that all types of bacteria required for the process can survive (and grow) in the system.

These design methods apply either

• the BOD ₅ and the excess sludge production occurring during its degradation or the resulting oxygen consumption or
• back to the COD . The COD can be exactly balanced in the sewage treatment plant. Both the wastewater pollution, the sludge production, the sludge content and the oxygen consumption can be expressed in the units of COD.

Nitrification and removal of nitrogen (denitrification)

Ammonium can be toxic to aquatic organisms (especially when converting ammonium to ammonia). In addition, nitrification takes place in water , which leads to oxygen consumption .

Furthermore, nitrate and ammonium are eutrophic (fertilizing) nutrients that can affect water.

For these reasons, nitrification and, in many cases, nitrogen removal is required.

Since not all of the nitrogen contained in the usual raw sewage is incorporated into the excess sludge by assimilation, two special process steps are necessary for nitrogen removal :

a) Nitrification: Oxidation of ammonium nitrogen and organically bound nitrogen to nitrate. This requires appropriate (slow-growing) bacteria (the nitrifying bacteria) and sufficient dissolved oxygen. Nitrification is very sensitive with regard to inhibitors and can lead to a pH value shift in poorly buffered water.

Nitrification takes place in the following steps:

1) Formation of nitrite :

${\displaystyle \mathrm {\ NH_{4}+1,5\ O_{2}\longrightarrow \ NO_{2}^{-}+2H^{+}+H_{2}O+Energie} }$

2) Formation of nitrate :

${\displaystyle \mathrm {\ NO_{2}^{-}+0,5\ O_{2}\longrightarrow \ NO_{3}^{-}+Energie} }$

in sum:

${\displaystyle \mathrm {\ NH_{4}+2\ O_{2}\longrightarrow \ NO_{3}^{-}+2H^{+}+H_{2}O+Energie} }$

This results in an oxygen consumption of 4.33 g O ₂ per g N. Nitrificant biomass increases by 0.24 g COD per g N (cell yield). One gram of COD (chemical oxygen demand) corresponds to 1.42 g of organic dry matter.

Nitrification is associated with the production of acid (H ⁺ ). This strains the buffer capacity of the water or a shift in the pH value may occur that affects the process.

b) Denitrification: Reduction of nitrate nitrogen to molecular nitrogen, which escapes from the wastewater into the atmosphere. This step can be carried out by the microorganisms commonly found in sewage treatment plants. However, these only use the nitrate as an electron acceptor (as an oxidant ) when there is no dissolved oxygen (anoxic conditions).

From a technical point of view, denitrification takes place in one step:

${\displaystyle \mathrm {\ 2\ NO_{3}^{-}+2\ H^{+}+10\ [H]\longrightarrow \ N_{2}+6\ H_{2}O} }$

One gram of NO ₃ -N is thus equivalent to 2.86 g of O ₂ as an oxidant and is available for the oxidation of substrate (e.g. carbon compounds from raw sewage). In order to allow denitrification to take place in the activated sludge process, an electron source, a reductant , must therefore also be present, which can sufficiently reduce nitrate to N ₂ . If there is too little substrate in the raw sewage, this can be added artificially (e.g. methanol ). In addition, the change in the H ⁺ concentration (pH value shift) that occurred during nitrification is corrected during denitrification . This is particularly important in poorly buffered water.

The nitrification and denitrification processes thus require a total of 1.5 g of O _{2 to} convert one gram of TKN ( total Kjeldahl nitrogen , org. N + NH ₄ -N) into N ₂ .

Nitrification and denitrification are in considerable contradiction with regard to the required environmental conditions. Nitrification requires oxygen (oxic conditions) and CO ₂ ( Nitrosomonas and Nitrobacter are chemolithoautotrophic microorganisms). Denitrification takes place only in the absence of dissolved oxygen (anoxic conditions) and with sufficient supply of oxidizable substances. This "dilemma" can be solved by the following methods:

Process for denitrification in single-stage sewage treatment plants using the activated sludge process

(A) simultaneous denitrification : intermittent operation by switching the ventilation on and off. A circulation unit may be required for thorough mixing or operation of a circulating pool. The oxygen content is regulated so that no dissolved oxygen is present in any part of the pool.

(B) In the upstream denitrification , the first tank is operated anoxically and the sludge / wastewater mixture is pumped back from the oxygen-rich second tank. This means that there is sufficient carbon from the feed to the first basin and nitrate from the return. The return rates are a multiple of the inflow.

(C) downstream denitrification : Nitrification is carried out in the first tank, denitrification in the second. There, however, the organic substances that were already inhaled in the first basin parallel to the nitrification with oxygen are missing. It is therefore necessary to add organic substances (e.g. methanol, molasses or acetate). This method is uncommon because of the high cost of substrate addition.

A particular problem is the removal of nitrogen in multi-stage systems. Since the organic substances are largely removed in the first stage and nitrification is carried out in the second, organic substances as electron sources for denitrification are missing in the third stage. This can only be solved through clever partial flow solutions and returns from the second to the first stage.

Removal of phosphate

→ Main article: Phosphorus elimination

In practice, phosphate can be removed chemically by precipitation with various iron and aluminum salts as well as biologically by incorporation into the biomass. Although the nutrient phosphate is sufficiently eliminated with the precipitation by metal salts, the modern management of sewage treatment plants strives for alternative ways of phosphate removal to save precipitants. Targeted operational management enables bacterial biomass to be cultivated that increasingly absorb phosphate in the biomass and thus remove it from the wastewater. In the course of the sludge treatment, however, it must be ensured that this phosphate does not redissolve in the sludge line (thickener, septic tank). Of the multitude of methods used in practice for biological phosphate removal, the more well-known include the following:

• Bardenpho method
• modified UTC procedure
• A2 / O procedure and the
• Phorodox method

In some of the processes, anaerobic and aerobic stages are combined with sludge recirculation and the final clarification in a slightly different order and with a different number of tanks. In the biological stage, depending on the process, a small, highly polluted tank can be integrated in front of the actual activation tank (selector) to support the absorption of phosphate in the biomass.

Operational problems

Bulking sludge

The separation of the biomass in the secondary clarifier is a fundamental component of the activated sludge process. In most cases, activated sludge that can be settled sufficiently and has a “good” flake structure is formed. Under certain circumstances, however, thread-like microorganisms gain a growth advantage and lead to extremely poor settling properties. Activated sludge with a sludge index of> 150 ml / g and a high degree of filament is called bulking sludge. As a result, there is massive loss of biomass from the secondary clarifier ("sludge drift"). The biomass content in the system drops, the cleaning performance decreases. Such a malfunction is particularly troublesome and can have very costly consequences.

A particular reason for the formation of bulky sludge in under-loaded sewage treatment plants is, in addition to other filamentous bacteria, often the microorganism Microthrix parvicella , which often leads to the foaming of the digester contents as an additional burden.

Common causes are nutrient deficiencies (N, P) and easily degradable or rotten wastewater (food industry). There is a relationship between the phosphorus-nitrogen ratio of the solids in the sludge and the sludge index . With a P / N ratio of <30%, the threadiness and sludge index increase until bulking sludge is formed, while this is generally no longer the case with a ratio> 35%. The causes and effects of bulky sludge formation can be combated by adding weighting precipitants such as an iron chloride sulphate solution (FeClSO ₄ ), shortening the length of time spent in the primary clarifier , adding nutrients and changing the process management (e.g. using selectors, see phosphorus removal).

Floating mud

Floating sludge removal with floating screw conveyors

In particular, by denitrification and development of bacteria having strong water repellent cell surface (eg. B. of " Nocardia "), it may in the secondary settling tank to form a floating sludge blanket (not to be confused with the "bulking", see above) may occur. In extreme cases, this can lead to several decimeter thick layers of mud and foam spreading over the pool. The thread-like bacteria that cause floating sludge often form when there is an increased influx of water-repellent substances and surface-active substances (surfactants, oils, fats, soaps), but these can also arise in the cleaning process itself. If the floating sludge remover is not functioning properly or the baffles in front of the drainage threshold are too small, undesired sludge drift can occur, which adversely affects the drainage values ​​of the sewage treatment plant. The formation of floating sludge can be suppressed both by adding flocculants and by removing sludge from the surface of the basin. The latter is achieved by ensuring sufficient sludge removal from the secondary clarifier and denitrification in the aeration tank.

For floating sludge removal in German-speaking countries, floating sludge removal systems are increasingly being used, in which screw conveyors arranged on the water surface convey the floating sludge to a slurping device. With the floating sludge pump arranged afterwards, the floating sludge is withdrawn from the pool via this slurping device.

power consumption

Sewage treatment plants should be dimensioned for the actual amount of wastewater. Excessive reserves lead to increased energy consumption. The oxygen content should be limited to the values ​​required for the process.

The anaerobic sludge stabilization can be used for digester gas production (fuel gas, mixture of mainly methane and carbon dioxide) and this can be used for heating and / or generating electricity .

Individual evidence

1. ↑ Handbook of the European Commission, in: Extensive Abwasserbehandlungsverfahren Guide , Internet version, p. 4.
2. ↑ Handbook of the European Commission, in: Extensive wastewater treatment process guide, Figure No. 4 , Internet version, p. 5.
3. ↑ G. Seibert-Erling, in: On the application of wastewater technical indicators in practical sewage treatment plant operation , Section 2.
4. ↑ G. Seibert-Erling, in: On the application of wastewater technical indicators in practical sewage treatment plant operation , Section 3.
5. ↑ Andreas Mohren, Min Nauendorf, in: Nitrification-Ammonia Oxidation , Practical Water Science (BSc.), WS 11/12.
6. ↑ NRW study, in: Status of phosphorus elimination in wastewater treatment , AZ IV – 9–042 423, May 31, 2002, table 2.1, p. 5.
7. ↑ NRW study, in: Status of phosphorus elimination in wastewater treatment , AZ IV – 9–042 423, May 31, 2002, pp. 10–12.
8. ↑ ^{a b} W. Maier, Kh. Krauth; Experience in bulking sludge control on sewage treatment plants; in: Korrespondenz Abwasser , 4, 1985, p. 245
9. ↑ UMTEC / feralco brochure, in: Combating floating sludge in sewage treatment plants , Internet version , p. 1.

swell

• ATV-DVWK-A 131 - Dimensioning of single-stage activated sludge systems . DWA, 2000, ISBN 978-3-933707-41-3 .