Environmental Water Treatment Knowledge: The Meaning and Role of Hydrolysis Acidification
Hydrolysis acidification is an important pretreatment process in wastewater treatment. Sitting between anaerobic and aerobic treatment, it is mainly used to improve the biodegradability of wastewater. Its core principle is to convert refractory macromolecular organics into readily degradable small molecules through two stages, hydrolysis and acidification [2] [8]. The process deliberately controls the reaction conditions, taking advantage of the fast growth and strong environmental adaptability of hydrolytic and acidifying bacteria to keep anaerobic treatment short of the methanogenic stage [9-10].
Unlike conventional anaerobic digestion, which aims at methane production, hydrolysis acidification takes the improvement of wastewater biodegradability (that is, raising the BOD/COD ratio) as its main goal, and generally does not pursue methane generation or actively suppresses it [8] [22].
Hydrolysis refers to the biochemical reaction that takes place outside the microbial cell before organic matter enters it: microorganisms release extracellular enzymes that convert complex insoluble polymers into simple soluble monomers or dimers. Acidification (acidogenic fermentation) then builds on this, with microorganisms fermenting small organic molecules into end products dominated by volatile fatty acids (VFAs) [10] [22]. In a real mixed microbial system, hydrolysis and acidification are closely coupled and hard to separate completely [22].
Anaerobic biological treatment of wastewater refers to the process in which, in the absence of molecular oxygen, anaerobic microorganisms (including facultative microorganisms) decompose and convert the various complex organics in wastewater into substances such as methane and carbon dioxide.
The anaerobic biochemical treatment process: the anaerobic degradation of macromolecular organics can be divided into four stages - hydrolysis, fermentation (or acidification), acetogenesis, and methanogenesis.
Hydrolysis can be defined as the process in which complex insoluble polymers are converted into simple soluble monomers or dimers.
2. Fermentation (or acidification) stage
Fermentation can be defined as a biodegradation process in which organic compounds act both as electron acceptors and as electron donors. In this process soluble organics are converted into end products dominated by volatile fatty acids, which is why the process is also called acidification.
Under the action of hydrogen-producing acetogenic bacteria, the products of the previous stage are further converted into acetic acid, hydrogen, carbonic acid, and new cell material.
In this stage, acetic acid, hydrogen, carbonic acid, formic acid, and methanol are converted into methane, carbon dioxide, and new cell material.
Because of their very large relative molecular mass, macromolecular organics cannot pass through the cell membrane and therefore cannot be used directly by bacteria. In the hydrolysis stage they are broken down into small molecules by bacterial extracellular enzymes. For example, cellulose is hydrolyzed by cellulase into cellobiose and glucose, starch is broken down by amylase into maltose and glucose, and protein is hydrolyzed by protease into short peptides and amino acids. These small-molecule hydrolysis products can dissolve in water and pass through the cell membrane to be used by bacteria. Hydrolysis is usually rather slow, and many factors, such as temperature, the composition of the organic matter, and the concentration of the hydrolysis products, may affect its rate and extent.
In the acidification stage, the small-molecule compounds above are converted inside the cells of acidifying bacteria into simpler compounds and secreted outside the cells. The vast majority of fermentative bacteria are strict anaerobes, but about 1% facultative anaerobes are usually present in the anaerobic environment, and these facultative anaerobes protect the strict anaerobes from oxygen damage and inhibition. The main products of this stage are volatile fatty acids, alcohols, lactic acid, carbon dioxide, hydrogen, ammonia, and hydrogen sulfide; the product composition depends on the anaerobic degradation conditions, the substrate types, and the microbial populations involved in acidification.
The hydrolysis stage is an unavoidable step in the degradation of macromolecular organics: before macromolecular organics can be used by microorganisms they must first be hydrolyzed into small molecules so that they can enter bacterial cells for further degradation. The acidification stage speeds degradation up, because it further converts the hydrolyzed small organic molecules into simple compounds and secretes them outside the cells. This is also why hydrolysis acidification is often used as a pretreatment unit in real industrial wastewater treatment projects.
Two widely recognized functions:
1. Improving wastewater biodegradability: macromolecular organics can be converted into small molecules.
2. Removing COD from wastewater: since these are heterotrophic microbial bacteria, they must draw nutrients from the environment, so part of the organic matter is inevitably degraded and used to build their own cells.
Hydrolysis acidification is one of the few technological creations with independent Chinese intellectual property rights. It was first proposed by Wang Kaijun in 1987, at a time when Chinese cities were in an early stage of development: municipal sewage had low biodegradability and economic conditions could not support the construction of high-investment, high-energy-consumption treatment plants. Around 1998 the Aksu wastewater treatment plant (capacity 60,000 t/d) adopted a hydrolysis plus improved SBR process, solving the technical bottleneck of scaling up hydrolysis tanks and laying the foundation for later large-scale application. [12]
In 2016 China issued the Technical Specifications for Wastewater Treatment Engineering with Hydrolysis Acidification Reactors (HJ 2047-2015), standardizing their design, construction, and operation. [13] Today the hydrolysis acidification process has become standard for refractory industrial wastewater, pharmaceutical and chemical wastewater, and dyeing and finishing wastewater, and has been applied in hundreds of municipal sewage projects and thousands of industrial wastewater projects. [12]
Compared with full anaerobic digestion, the hydrolysis acidification process requires no sealed tank, no agitator, and no three-phase separator, which lowers both construction cost and operation and maintenance cost [11] [22]. The process reacts quickly, has a short hydraulic retention time, produces effluent without the foul odor of anaerobic fermentation, and can treat sewage and sludge in one step, reducing sludge quantity [11] [15]. Combined with a downstream aerobic process, it saves overall operating costs, improves aerobic treatment efficiency, reduces excess sludge production, and buffers fluctuations in influent load [15] [22].
Reactor types for hydrolysis acidification mainly include the upflow hydrolysis reactor, the composite hydrolysis reactor, and the completely mixed hydrolysis reactor [9-10]. Anaerobic baffled reactors and anaerobic contact reactors can also be used [9] [15].
A hydrolysis acidification tank generally comprises the tank body, a water distribution system, a packing system (such as fixed-bed flat-plate packing), a sludge discharge system, and an optional agitation device. Uniform water distribution is critical to treatment performance, and the influent system must serve both distribution and mixing functions [8] [15].
The hydraulic retention time (HRT) is generally 2.5-4.5 hours for municipal sewage and can be extended beyond 8 hours for refractory industrial wastewater (such as pharmaceutical and dyeing wastewater) [9] [15-16]. The upflow velocity is generally controlled at 0.8-1.8 m/h [9] [16], and the influent COD concentration should preferably be below 1,500 mg/L [9].
Key factors affecting hydrolysis acidification performance include substrate type and particle size, volumetric loading, the water distribution system, upflow velocity, and pH (suitable range 4.0-9.0) [10] [16] [22].
Hydrolysis acidification relies mainly on fermentative bacteria (hydrolytic-acidifying bacteria). These bacteria are highly diverse, metabolically powerful, fast-reproducing, and highly adaptable to external conditions [9] [17] [22].
Studies show that hydrolysis acidification improves the biodegradability of wastewater containing complex organics such as heterocyclic and aromatic hydrocarbons, decomposing macromolecular organics by breaking polycyclic structures through the ring-opening enzyme system of the microorganisms [7]. Based on the difference in growth rates between methanogens and hydrolytic-acidogenic bacteria, the process keeps the reaction within the hydrolysis and acidification stages of anaerobic treatment, and it was developed step by step through repeated experiments and theoretical analysis [11].
The process is widely applicable and effective for many kinds of poorly biodegradable or high-concentration organic wastewater, including municipal sewage, textile dyeing wastewater, pharmaceutical wastewater, papermaking wastewater, brewery wastewater, petrochemical wastewater, fine chemical wastewater, slaughterhouse wastewater, food processing wastewater, starch wastewater, acrylic fiber wastewater, aniline wastewater, photographic film wastewater, fermentation pharmaceutical wastewater, traditional Chinese medicine wastewater, pulping wastewater, and municipal landfill leachate [9-11] [15] [19].
In practice, hydrolysis acidification is often used as a pretreatment unit in combination with other processes to form an efficient treatment system. Typical combinations include hydrolysis acidification plus aerobic treatment, hydrolysis acidification plus UASB (upflow anaerobic sludge blanket) plus SBR (sequencing batch reactor), coagulation plus hydrolysis acidification plus two-stage contact oxidation, and hydrolysis acidification plus reaction sedimentation plus contact oxidation [10] [19].
Real engineering cases show that hydrolysis acidification can significantly improve wastewater biodegradability. For example, after hydrolysis acidification of a pharmaceutical production wastewater the BOD5/COD ratio rose from 0.152-0.218 to 0.436-0.496; a brewery wastewater treated by a hydrolysis plus biological contact oxidation process saw its BOD5/CODCr ratio rise from 0.51 to 0.72; a dyeing wastewater treated by a hydrolysis acidification plus aerobic process had its BOD/COD ratio raised to 0.3-0.45; and at a municipal wastewater treatment plant using a hydrolysis plus aerobic process the hydrolysis reactor achieved a certain removal rate for BOD5, CODCr, and SS while improving sewage biodegradability [9].
The core impact of the hydrolysis acidification process is that, by converting refractory macromolecular organics into readily degradable small molecules, it significantly improves wastewater biodegradability, that is, raises the BOD/COD ratio [7-10]. The process effectively removes part of the COD and suspended solids, lowering the organic load and oxygen demand of downstream aerobic treatment and thus saving overall operating costs [10] [14-15] [22]. Sludge production is far lower than in aerobic processes (about 1/10 to 1/6 of an aerobic process), and the sludge is already highly mineralized and easy to handle; meanwhile excess sludge from the downstream aerobic stage can be returned to the hydrolysis stage for further digestion, helping to reduce total system sludge production [15] [18] [20-22]. The hydrolysis acidification tank strongly resists influent load shocks and provides more stable influent conditions for downstream units [10] [14-15] [19] [22]. Compared with full anaerobic or aerobic processes, it offers lower investment, lower operating cost, and a smaller footprint [11] [14-15] [22].
The hydrolysis acidification process serves mainly as a pretreatment unit. Its purpose is to convert non-dissolved organics in wastewater into dissolved organics, and in particular to convert refractory macromolecules into readily biodegradable small molecules, thereby improving wastewater biodegradability (the BOD5/COD ratio) and creating favorable conditions for subsequent aerobic or anaerobic biological treatment [9-10].
Current research focuses on the resource recovery and volume reduction of organic waste and on the treatment of high-concentration industrial wastewater, and patented technologies exist for enhanced hydrolysis acidification methods targeting specific wastewaters such as chemical wastewater [23].
Emerging research focuses on optimizing the hydrolysis acidification of biomass such as straw by directionally regulating microbial communities (for example, reshaping lactate- and acetate-producing consortia) in order to improve the production efficiency of medium-chain fatty acids [24-25]. Researchers have used artificial intelligence to analyze metabolic pathways and identify key enzymes and associated microorganisms, raising the production potential of medium-chain fatty acids through directed regulation [24].
