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Anaerobic treatment converts organic matter in wastewater into a small quantity of sludge and a large quantity of biogas (CH₄ + C0₂), while leaving some pollution unremoved. In contrast, aerobic processes produce a large quantity of sludge and no biogas, while also leaving some pollution, though, less than that in the anaerobic treatment. The main advantages of anaerobic treatment are:

1. Low operating costs

2. Less space requirements

3. Energy recovery (biogas production)

4. Low sludge production.

Anaerobic stabilization occurs in three stages. In the first stage, complex organics (cellulose, proteins, lipids, etc.) are broken down by extracellular enzymes into soluble organic fatty acids, alcohols and carbon dioxide. In the second stage, the products of the first stage are converted into various organic acids and alcohols by acid-forming bacteria. In the third stage

The biological degradation of complex organic matter takes place in several consecutive biochemical steps, each performed by different groups of specialized bacteria. In practice, the acetogenic and methanogenic phases are the rate-limiting steps. On the other hand, the generation of methane gas can only happen as fast as methane-forming bacteria receive their substrate. Methane-forming bacteria only use acetic acid (CH₃COOH), hydrogen gas (H₂) and carbon dioxide (C0₂) as substrate.

It is well known that the rate-limiting stage (i.e., the stage which is the slowest) controls the process. In anaerobic process methane-forming bacteria reproduce very slowly. These methane-forming bacteria are also very sensitive to environmental factors such as pH, alkalinity, temperature and toxins. Therefore, controlling these factors is very important to control the process of digestion.

Anaerobic digestion of wastewater sludges

In primary and secondary treatment processes, a significant fraction of the removed BOD in extracted as sludge. This sludge must be treated further before its safe disposal. One of the most widely employed sludge treatment technologies is anaerobic digestion. Here a large fraction of the organic matter in the sludge in converted to carbon dioxide and methane by microorganisms that act in the absence of oxygen.

The treatment of wastewater sludges consists of two main phases. In the first phase, the objective is to separate the water from the sludge by adopting thickening and dewatering processes. The second phase is known as sludge stabilization. There are three primary objectives of sludge stabilization:

1. To reduce the level of pathogens in the residual solids.

2. To eliminate offensive odors.

3. To reduce potential for putrefaction.

It consists of two identical reactors and the first one is sealed to maintain anaerobic condition. The second one may be either sealed for additional gas collection or may be open to the atmosphere. The input to the digester in typically a mixture of solids from primary settling tank, wasted microbes from secondary settling tank and surface from both primary and secondary settling tanks. These sludges are added to the first reactor continuously or intermittently. This first stage reactor is heated and mixed to accelerate the biological conversion. After a typical residence time of 10-20 days, the mixed digested sludge passes to the second reactor. Here it is allowed for further digestion without mixing and heating. Settled sludge is removed from the second reactor, either intermittently or continuously. The removed sludge in then dewatered and disposed the supernatant liquid may be recycled to the beginning of the wastewater treatment plant, if necessary.

The significance of the microbiology of anaerobic digestion is manifest in optimum system design and operation. Process design should be directed towards maintaining a large, stable population of methane-forming bacteria. Typically, inadequate system design or operation will result in relatively high volatile acid accumulation in the digester. The excess acid accumulation may create an imbalance between the population of acid-forming and methane-forming bacteria. In a severe case excessive high volatile acid production will depress the pH to a level that essentially stops the activity of Methanogenesis entirely. Stable performance can be achieved through careful consideration of the fundamentals of anaerobic treatment. The following environmental and operational parameters are to be considered while designing and operating the system.

Environmental parameters:

a. pH

b. Alkalinity

c. Temperature

d. Nutrient content

e. Toxic compounds

2. Operational parameters:

a. Solids retention time

b. Substrate characteristics

– Concentration

– Composition

– Biodegradability

Tertiary/Advanced Treatment

Tertiary and/or advanced wastewater treatment is employed when specific wastewater constituents which cannot be removed by secondary treatment must be removed. Individual treatment processes are necessary to remove nitrogen, phosphorus, additional suspended solids, refractory organics, heavy metals and dissolved solids. Because advanced treatment usually follows high-rate secondary treatment, it is sometimes referred to as tertiary treatment.