Anaerobic Digestion in Four Stages: Using the Chemistry to Diagnose a Reactor That Is Drifting

Two reactor families

The anaerobic process works by creating, under anaerobic conditions, the nutritional and environmental conditions anaerobic microorganisms require, then using them to decompose the organic matter in wastewater. The products fall mainly into two kinds, and knowing which kind a reactor is producing is often the fastest way to tell whether it is healthy. Grouped by the aggregation form of the microorganisms, anaerobic biological treatment divides into the anaerobic activated sludge process and the anaerobic biofilm process. The activated sludge branch includes the conventional digester, the anaerobic contact digester, the upflow anaerobic sludge blanket (UASB) and the expanded granular sludge bed (EGSB); the biofilm branch includes the anaerobic biological filter, the anaerobic fluidized bed and the anaerobic rotating biological contactor.

Stage one: hydrolysis

Hydrolysis is the conversion of complex insoluble polymers into simple soluble monomers or dimers. Because macromolecular organics have a very large relative molecular mass they cannot pass through the cell membrane, so they are broken into small molecules by bacterial extracellular enzymes first: 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. Only these small-molecule products dissolve in water and cross into the cell.

Hydrolysis is usually rather slow, and is therefore regarded as the rate-limiting stage in the anaerobic degradation of wastewater containing macromolecular organics or suspended solids. Temperature, the composition of the organic matter and the concentration of hydrolysis products all influence rate and extent, captured by p=po/(1+Kh.T), where p is the concentration of degradable non-dissolved substrate in g/L, po the initial concentration of non-dissolved substrate, Kh the hydrolysis constant in d^-1 and T the retention time in d.

Stage two: fermentation, or acidification

Fermentation is a biodegradation process in which organic compounds act as both electron acceptors and electron donors, converting soluble organics into end products dominated by volatile fatty acids, which is why the stage is also called acidification. Inside the fermentative bacteria, the small molecules from hydrolysis are converted into simpler compounds and secreted outside the cell. The vast majority of these bacteria are strict anaerobes, but about 1% facultative anaerobes are usually present, and they protect strict anaerobes such as methanogens from oxygen damage and inhibition.

Main products are volatile fatty acids, alcohols, lactic acid, carbon dioxide, hydrogen, ammonia and hydrogen sulfide, with the actual composition depending on degradation conditions, substrate types and the microbial populations involved. Because the acidifying bacteria also build new cell matter, anaerobic treatment of non-acidified wastewater produces more excess sludge.

Acid tolerance is the practical lever here. Acidification still proceeds when pH falls to 4, whereas methanogenesis does not, so a falling pH reduces methane generation and hydrogen consumption and shifts the composition of the acidification end products. Watching that shift is cheaper than waiting for gas production to collapse.

Stage three: acetogenesis

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. The stoichiometry is what engineers actually check:

CH3CHOHCOO-+2H2O —> CH3COO-+HCO3-+H++2H2 ΔG'0=-4.2KJ/MOL
CH3CH2OH+H2O> CH3COO-+H++2H2O ΔG'0=9.6KJ/MOL
CH3CH2CH2COO-+2H2O> 2CH3COO-+H++2H2 ΔG'0=48.1KJ/MOL
CH3CH2COO-+3H2O> CH3COO-+HCO3-+H++3H2 ΔG'0=76.1KJ/MOL
4CH3OH+2CO2> 3CH3COO-+2H2O ΔG'0=-2.9KJ/MOL
2HCO3-+4H2+H+>CH3COO-+4H2O ΔG'0=-70.3KJ/MOL

Stage four: methanogenesis

The conversion of acetic acid, acetate, carbon dioxide and hydrogen into methane is carried out by two physiologically different groups of methanogens. One converts hydrogen and carbon dioxide into methane and accounts for about 1/3 of the total; the other produces methane by decarboxylating acetic acid or acetate and accounts for about 2/3. The main intermediate product in the formation of methane is methyl-coenzyme M (CH3-S-CH2-SO3-).

The most important methanogenic reactions are:

CH3COO-+H2O>CH4+HCO3- ΔG'0=-31.0KJ/MOL
HCO3-+H++4H2>CH4+3H2O ΔG'0=-135.6KJ/MOL
4CH3OH>3CH4+CO2+2H2O ΔG'0=-312KJ/MOL
4HCOO-+2H+>CH4+CO2+2HCO3- ΔG'0=-32.9KJ/MOL

In this stage acetic acid, hydrogen, carbonic acid, formic acid and methanol are converted into methane, carbon dioxide and new cell material. Note that some books divide anaerobic digestion into three stages, merging the first two into a hydrolysis acidification stage; treating it as four reflects the process more clearly.

Choosing between the reactor families

The choice between an anaerobic activated sludge process and an anaerobic biofilm process comes down to what the biomass has to hold on to. Where the wastewater carries few suspended solids and the load is soluble, UASB and EGSB let the biomass grow into dense granules that stay in the reactor without any carrier, and EGSB pushes the upflow velocity higher so the bed expands and contact improves. Where the stream carries solids that would wash granules out, or where start-up speed matters more than peak loading, an anaerobic biological filter or anaerobic fluidized bed gives the microorganisms a fixed surface to colonise. The anaerobic contact digester sits between the two, separating and returning biomass the way an aerobic activated sludge plant does.

Running all four at once

Reaction rates of the four stages vary with the nature of the wastewater. Where pollutants such as cellulose, hemicellulose, pectin and lipids dominate, hydrolysis readily becomes the rate-limiting step. Simple sugars, starch, amino acids and ordinary proteins are rapidly decomposed by microorganisms, so for wastewater containing those organics methanogenesis tends to become the rate-limiting stage instead.

Inside an anaerobic reactor all four stages proceed simultaneously and maintain a degree of dynamic balance. That balance is the thing to protect. Once it is upset by external factors such as pH, temperature or organic loading, the methanogenic stage is inhibited first, which leads to accumulation of lower fatty acids, abnormal changes in the anaerobic process, and in the worst case brings the whole digestion process to a standstill. Reading which stage has slipped, from volatile fatty acid accumulation, gas composition or alkalinity, is the fastest route to a diagnosis.