Raising the BOD/COD Ratio Before Aerobic Treatment: What Hydrolysis Acidification Really Does
Refractory industrial wastewater fails in the aerobic tank for a boring reason: the molecules are too big. Anything with a relative molecular mass above what a cell membrane will pass has to be cut down outside the cell first, and if that does not happen upstream the aerobic stage simply churns. Hydrolysis acidification exists to do that cutting, and it stops there deliberately instead of running through to methane.
Deliberately stopping short of methanogenesis
Full anaerobic digestion runs four stages: hydrolysis, fermentation or acidification, acetogenesis and methanogenesis. Hydrolysis acidification holds the reaction in the first two. The reason is kinetic rather than philosophical. Hydrolytic and acidifying bacteria grow fast and tolerate a wide range of conditions, whereas methanogens are slow and fussy. Control the environment so the fast population dominates and the process lands on improved biodegradability, that is a higher BOD/COD ratio, without pushing for gas.
In the hydrolysis step, microbes release extracellular enzymes that turn complex insoluble polymers into soluble monomers and dimers. Cellulose becomes cellobiose and glucose via cellulase, starch becomes maltose and glucose via amylase, and protein becomes short peptides and amino acids via protease. Only then can the material cross the cell membrane. Acidification follows inside the acidifying bacteria, which ferment those small molecules to end products dominated by volatile fatty acids. In a real mixed culture the two steps are so tightly coupled that separating them is largely a theoretical exercise.
About 1% of the population in an anaerobic environment is facultative, and that minority earns its place by scavenging oxygen and shielding the obligate anaerobes. Alongside the volatile fatty acids the stage also yields alcohols, lactic acid, carbon dioxide, hydrogen, ammonia and hydrogen sulfide, with the exact mix set by substrate, conditions and the microbial community present.
Two payoffs, one of them often overlooked
The headline benefit is biodegradability: the BOD/COD ratio rises and the downstream aerobic plant suddenly has something it can eat. The second benefit is quieter but real. These are heterotrophic organisms that must build cell material from what they consume, so a slice of the COD is genuinely removed rather than merely reshaped. Effluent leaves with lower organic load and lower oxygen demand ahead of the aeration tank, and the aerobic stage produces less surplus sludge.
Field numbers bear it out. A pharmaceutical wastewater moved from a BOD5/COD ratio of 0.152-0.218 to 0.436-0.496 after hydrolysis acidification. Brewery effluent treated by hydrolysis plus biological contact oxidation went from a BOD5/CODCr ratio of 0.51 to 0.72. A dyeing wastewater on hydrolysis acidification plus aerobic treatment was lifted to a BOD/COD of 0.3-0.45. Municipal plants report the same directional effect, with the hydrolysis reactor taking a measurable bite out of BOD5, CODCr and suspended solids at the same time.
Reactor form and the parameters that matter
Configurations fall into three families: the upflow hydrolysis reactor, the composite hydrolysis reactor and the completely mixed hydrolysis reactor, with anaerobic baffled reactors and anaerobic contact reactors also used. A typical tank comprises the shell, a water distribution system, a packing system such as fixed-bed flat-plate packing, a sludge discharge system and optionally an agitator. Uniform distribution is not a detail. The influent system has to distribute and mix at the same time, and poor distribution shows up directly as lost performance.
Hydraulic retention time (HRT) is usually 2.5-4.5 hours on municipal sewage and can be stretched past 8 hours on genuinely refractory industrial streams such as pharmaceutical and dyeing effluent. Upflow velocity is held around 0.8-1.8 m/h. Influent COD is best kept below 1,500 mg/L. Beyond those, the variables that move the needle are substrate type and particle size, volumetric loading, the distribution system, upflow velocity and pH, which has a workable band of 4.0-9.0.
What it saves compared with full anaerobic
No sealed tank, no agitator, no three-phase separator: capital and O&M both come down. The reaction is quick, retention is short, and the effluent does not carry the smell of anaerobic fermentation. Sewage and sludge can be treated in one step, which cuts sludge volume, and surplus sludge from the aerobic stage downstream can be returned to the hydrolysis stage for further digestion. Reported sludge production is roughly a tenth to a sixth of an aerobic process, and what is produced is already well mineralised and easy to handle. Add the buffering of influent shocks and the downstream units see a far steadier feed.
Where it fits in a flowsheet
The process is rarely the whole answer; it is the front end. Common pairings are 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. It has become standard on refractory industrial wastewater, pharmaceutical and chemical effluent and dyeing and finishing wastewater, and has been applied in hundreds of municipal projects and thousands of industrial ones.
There is a distinctly Chinese lineage here. The process was first proposed by Wang Kaijun in 1987, at a point when municipal sewage was poorly biodegradable and there was no budget for high-investment, high-energy plants. The 60,000 t/d Aksu plant adopted hydrolysis plus a modified SBR around 1998 and broke the scale-up bottleneck for large hydrolysis tanks. China then issued the Technical Specifications for Wastewater Treatment Engineering with Hydrolysis Acidification Reactors (HJ 2047-2015) in 2016, which fixed design, construction and operating practice.