Anaerobic Digestion Wastewater Treatment

The process of purification of domestic and industrial wastewater is accompanied by the release of a large amount of sediment. Untreated, they could cause pollution or contamination of the environment due to the high concentration of pollutants in them.

Untreated sludge contains a huge amount of microorganisms, as well as organic components that quickly rot. To prevent the possibility of contamination or contamination, the separated sludge is subjected to additional treatment. Among the main processes in their processing. Aimed at breaking down the organic substances that cause foul-smelling gases and infections, is their biochemical stabilization. In it, microorganisms convert complex organic substrates, such as polysaccharides, lipids, and proteins, into simpler inorganic products. Anaerobic biochemical processes are generally used for sludge stabilization. Depending on the chosen method, the type of facility where the stabilization process will take place is also determined. Digesters are usually used for anaerobic stabilization.

Anaerobic stabilization For Wastewater Treatment

In this method of stabilization of primary and secondary sludges, bioreactors and open digesters are usually used. The active organic load, and the amount of sludge are reduced by the biological degradation of the organic material contained in the absence of oxygen.

Degradation in bioreactors is carried out at mesophilic (30-38 °C) or thermophilic (49-57 °C) temperature conditions, and usually lasts 20-30 days. Methane is produced as a by-product, which can be used for energy production. Rotted sludge no longer emits an odor because the organic matter originally contained in it has largely decomposed.

After anaerobic digestion in bioreactors, the organic matter of the sludge is reduced by 30-40%. And in well-functioning facilities, it is expected to obtain a stabilized residue after passing the sludge through the bioreactors.

Pathogens

It is highly likely that this residue contains some pathogens. Therefore, this requires temperatures for their destruction much higher than those in anaerobic digesters. In the case of subsequent utilization of the rest of the sediments. It is mandatory to carry out a full physicochemical and microbiological analysis. This is in order to prove their suitability for use in agriculture, land reclamation, or storage.

Often, for decontamination, additional treatment with quicklime is also applied. Which increases the cost of treating the sludge by anaerobic digestion.

By means of various measures; a better degradation of the material can be achieved and thus the volume of the sludge can be reduced. As well as a greater amount of gas and less slag can be obtained. They also positively influence the further utilization of sludge.

Sludge homogenization increases viscosity by up to 40%. Chemical oxygen demand (COD) by 130% and thus improves gas production by 10-30%. Which in turn reduces unwanted greenhouse gas emissions. It also reduces the costs of cleaning the installations from flocculants and residues.

Waste and wastewater treatment

Especially effective at processing organic material, anaerobic digestion is frequently used to remediate wastewater, sewage sludge, and industrial effluent. Anaerobic digestion is a quick and easy procedure. This procedure can significantly cut down on the quantity of organic waste that would otherwise end up in landfills, incinerators, or the ocean.

The use of anaerobic digestion of wastewater is a strategy for lowering waste quantities and producing usable byproducts has expanded due to pressure from environmental legislation on solid waste disposal methods in developed countries. It can be used to process the source-separated portion of municipal waste. Alternatively, to process leftover mixed municipal waste when paired with mechanical sorting devices. Mechanical biological treatment plants are the name given to these establishments.

Avoiding temperature fluctuations

The temperature in the bioreactor should not fall below 38 °C and should be maintained within constant limits. The implementation of additional, artificial warming of the sediments ensures an accelerated course of the processes of mineralization of organic substances. For this purpose, various methods are used, which can be divided into two main groups – with direct and indirect heating.

The methods of direct heating of sludge are based on the use of superheated steam or hot water, which are supplied directly – inside the digester itself. In recent years, as a result of achieving very good results, preference is given to the use of low-pressure steam (up to 0.5 atm) with a temperature of 110 °C, which is fed into the sludge line for fresh sludge. The method makes it possible to heat a small volume in a short time, which is why it is also considered very suitable for the pasteurization of sediments.

Indirect heating uses the principle of heat exchange. The characteristic of indirect heating is that a heat carrier (hot steam or hot water) circulates through a closed system consisting of a heat exchanger and a heater. The heat exchangers used to heat the sludge are internal and external, depending on their location relative to the bioreactor.

Chemical disintegration

In this method, the structure of the sediments is changed through chemical processes and thus biodegradation is facilitated. Chemical disintegration mainly involves inducing precipitation using iron salts, which reduces phosphorus and thus improves dewatering; the use of polymer-based precipitants; the addition of slaked lime to sanitize the sludge and stabilize the acid base.

In the latter method, the raw dewatered sludge can be mixed with lime in an amount sufficient to raise the pH above 12 and held under these conditions for 2 hours in order to maintain the temperature of the mixture above 70 °C. Lime consumption can reach up to about 500 kg per ton of dry matter in the sludge. Instead of lime, cheaper dust from cement kiln filters can be used. After lime treatment, the dry matter content in the sludge increases to 37%.

Conclusion

The continuous stirring of the sediments is carried out by means of pumps, hydraulic elevators, propeller stirrers, etc. Among the preferred methods is the use of propeller stirrers, since they achieve optimal results, and the consumption of electricity is many times lower compared to that typical of hydro elevators, for example.

Mixing of the raw with the rotting sludge is increasingly practiced, which is carried out by means of the gas released from the bioreactor itself and collected in its dome. Several methods are used to mix sludge using methane. Among those that have proven to be effective is the supply of a gas jet at the base of the digester, where an intense mixing flow occurs from the center to the periphery.

Gas is supplied to the digester by means of diffusers or vertical pipes. It should be borne in mind, however, that the effectiveness of this method depends largely on the ratio between the diameter of the rotator and its height. Typically, in the process of sizing the devices used for agitating the sludge, it is intended that the transfer of all the sludge in the facility takes place during the interval of 5 to 10 hours.