Hydrolysis in chemistry refers to a type of reaction between a compound and water. In wastewater treatment, hydrolysis refers to the biochemical reaction that occurs outside the cell before the organic substrate enters the cell. Hydrolysis is the process by which complex insoluble polymers are converted into simple soluble monomers or dimers. Polymer organic compounds, due to their enormous relative molecular weight, cannot penetrate cell membranes and therefore cannot be directly utilized by bacteria. They are first transformed into small molecules by the hydrolysis of extracellular enzymes in bacteria. The most typical feature of this stage is that the site of biological reactions occurs outside the cell, and microorganisms complete biocatalytic oxidation reactions (mainly including chain breakage and water solubility of macromolecules) by releasing extracellular free enzymes or fixed enzymes connected to the cell wall.

Acidification is a typical fermentation process, that is, the acid producing fermentation process. Acidification is the biodegradation process of organic substrates as both electron acceptors and electron donors. During the acidification process, soluble organic matter is converted into terminal products mainly composed of volatile acids.

In a mixed microbial system under anaerobic conditions, even if the conditions are strictly controlled, hydrolysis and acidification cannot be completely separated. This is because hydrolysis bacteria are actually fermentation bacteria with hydrolysis ability. Hydrolysis is an energy consuming process, and fermentation bacteria invest energy in hydrolysis to obtain water-soluble substrates that can be fermented, and obtain energy through intracellular biochemical reactions, while excreting metabolic products (mainly various organic acids under anaerobic conditions). If both insoluble and soluble organic matter are present in the wastewater, hydrolysis and acidification are inseparable simultaneously. If acidification causes a significant decrease in pH value, it is not conducive to hydrolysis.

The anaerobic fermentation process for producing biogas can be divided into four stages: hydrolysis stage, acidification stage, acetylation stage, and methane stage. Hydrolysis acidification process is the process of controlling anaerobic treatment in the first and second stages with shorter reaction time, that is, hydrolyzing insoluble organic matter into soluble organic matter, converting difficult to biodegrade macromolecular substances into easily biodegradable small molecule organic substances.

1.1The influencing factors of hydrolysis acidification

a) The type of matrix and particle size

The difficulty of hydrolysis varies depending on the substrate. The type of substrate has a significant impact on the rate of hydrolysis and acidification process. Under the same conditions, the hydrolysis rate of fats, proteins, and polysaccharides gradually increases; For organic compounds of the same type, those with higher molecular weight are more difficult to hydrolyze than those with lower molecular weight; In terms of molecular structure, the degree of hydrolysis difficulty is linear chain structure>branched chain structure>cyclic structure, and monocyclic compounds are more prone to heterocyclic compounds. The particle size of pollutants also has a significant impact on the hydrolysis rate. The larger the particle size, the smaller the specific surface area per unit weight, making it more difficult to hydrolyze. Therefore, for wastewater or sludge with high concentrations of large organic pollutants, they should be first crushed before entering the hydrolysis tank to accelerate the hydrolysis (acidification) rate.

b) Volumetric load

Volume load is one of the important process parameters in the hydrolysis process, which reflects the comprehensive impact of influent concentration and residence time on anaerobic processes. For hydrolysis reactors, the design value of volumetric load is relatively low, increasing the hydraulic retention time can prolong the contact time between pollutants and hydrolysis microorganisms, increase the dissolved COD concentration, and make hydrolysis more complete. For urban sewage, hydrolysis reaction can be completed in a short time, and the volumetric load can be relatively high; For wastewater with a large proportion of industrial wastewater, the volumetric load needs to be designed based on the nature of the wastewater.

c) Water distribution system

One of the important conditions for the good operation of the hydrolysis tank is to ensure sufficient contact between sludge and wastewater, so the water distribution system at the bottom of the system should be as uniform as possible. The water distribution system of the hydrolysis reactor is a crucial design system. In order to ensure uniform inlet water at the bottom of the reactor, it is necessary to use a distribution device that evenly distributes the inlet water to multiple inlet points.

d) Upward flow rate

To ensure sufficient contact between sludge and water in the hydrolysis reactor and the quality of the effluent, the upward flow rate of the hydrolysis tank should be controlled within a certain range. When the rising flow rate is low, a large amount of denser activated sludge deposits at the bottom of the hydrolysis tank. During the process of sewage rising, the mud water cannot fully contact the reaction, resulting in poor removal efficiency. When the rising flow rate is too high, it will cause a significant loss of activated sludge in the hydrolysis tank. The effluent carrying mud not only causes toxicity to the microorganisms in subsequent aerobic biochemical treatment, but also cannot guarantee the removal efficiency of the hydrolysis tank.

1.2Advantages of hydrolysis acidification process

The hydrolysis and acidification stage mainly utilizes fermentation bacteria, which have a wide variety of types, strong metabolic ability, fast reproduction speed, and strong adaptability to the external environment.

Compared with separate aerobic processes, the combination of hydrolysis acidification process and aerobic process has the following advantages:

1. The hydrolysis and acidification process has low operating costs, and its removal of organic matter in wastewater can also save the oxygen demand of the aerobic section, thereby saving the overall operating costs of the process;

2. The hydrolysis and acidification process has caused significant changes in the quantity and physicochemical properties of organic compounds in wastewater, making it more suitable for subsequent aerobic treatment and improving the efficiency of aerobic treatment;

3. The mud production of hydrolysis acidification process is much lower than that of aerobic process, and it is highly mineralized and easy to treat;

4. The hydrolysis acidification process can play a buffering role in the change of influent load, thereby creating relatively stable influent conditions for aerobic treatment; 

The use of hydrolysis tanks has the following advantages over anaerobic tanks (digestion tanks) throughout the entire process:

1. The products in the hydrolysis and acid production stages are mainly small molecule organic compounds, with generally good biodegradability. Therefore, hydrolysis tanks can change the biodegradability of raw wastewater, thereby reducing reaction time and treatment energy consumption.

2. The degradation of solid organic matter can reduce the amount of sludge, and its function is the same as that of a digestion tank. The process only produces a small amount of biologically activated sludge that is difficult to anaerobic degrade, thus achieving one-time treatment of sewage and sludge without the need for a frequently heated medium temperature digestion tank.

3. No need for a sealed pool, no need for a mixer, no need for a three-phase separator for water, gas, and solid, reducing cost and facilitating maintenance. Due to these characteristics, structures suitable for large, medium, and small sewage treatment plants can be designed.

4. Before the completion of the second stage of reaction control, the effluent should have no adverse odor of anaerobic fermentation, improving the environment of the treatment plant.

5. The first and second stages react quickly, so the hydrolysis tank has a small volume, which is equivalent to the initial sedimentation tank, saving infrastructure investment.