Anaerobic Digestion for Environmental Processes

Term project by Edmond White, May 1997

Introduction

As I looked through the different web pages for Biochemical Engineering I realized that for the amount of material covered in the course it often becomes difficult to get very specific on one type of process. I decided to cover some of the specifics for Anaerobic Digestion used to stabilize waste water treatment sludge.

The treatment of wastewater and sewage creates a problem; that of the disposal of the by-products of this process. For an engineer, this problem may create the single most complex and costly process of the whole wastewater treatment process. Municipal wastewater treatment plants generate sludges as a by-product of the physical, chemical and biological processes used in the treatment of sewage. Generally, this sludge must be subject to some form of treatment in order to alter its character. It may then be disposed of without creating health problems or further hindrance. This treatment has many objectives. First, to reduce the volume of excess material by eliminating the liquid portion of the sludge. The second goal is to decompose the highly putrescible organic matter into relatively stable or inert organic and inorganic compounds; thus allowing water to separate more easily. By using anaerobic digestion in the treatment of wastewater sludge, the overall cost of sewage treatment is reduced and it also furnishes a considerable power supply. Although many sludge stabilization methods exist, anaerobic digestion is unique for it has the ability to produce a net energy gain in the form of methane gas, it optimizes cost effectiveness and minimizes the amount of final sludge disposal, thus decreasing the hazards of wastewater and sewage treatment by-products.

Background

The first step in the wastewater treatment process is pretreatment. The purpose of this step is to remove large solids and grits through screening. The screenings by-products are disposed of separately from the other wastewater sludges. The next step in the wastewater treatment process is primary treatment, which involves the use of clarifiers and sedimentation tanks to settle particulates in the wastewater. Primary treatment removes approximately 50 to 65 percent of the suspended solids and 30 to 40 percent of the biochemical oxygen demand (BOD) from the wastewater. The sludge removed from this process contains mostly organic matter that is highly putrescible.

Following primary treatment, the wastewater is subject to secondary treatment. In nearly all municipal wastewater treatment facilities aerobic microorganisms are used to biologically remove the remaining BOD and suspended solids. This effluent then passes through a secondary clarifier, producing a sludge consisting of nearly 90 percent organic matter. Overall, this sludge is composed of approximately 2 to 4 percent solids and if not treated correctly, it becomes highly odiferous.

In most cases, the sludges from primary and secondary treatment are combined and undergo another form of treatment before their disposal. First, the sludge is thickened, by gravity or floatation, removing as much water as possible. Thickening may reduce the amount of sludge to as little as half of the original volume. The liquid effluent from this process is recycled back to the beginning of the treatment process. Once this has been accomplished, the sludge is subject to some form of stabilization. This process converts the organic solids to more inert forms so that they may be disposed of without causing health problems or further difficulties.

The Anaerobic Digestion Process

An anaerobic treatment system is a complex three-step process that produces methane gas (in addition to other products) from the biological digestion of sewage waste. The first stage is the hydrolysis of lipids, cellulose, and protein. Extracellular enzymes produced by the inhabiting bacteria breakdown these macromolecules into smaller and more digestible forms. Next, these molecules are decomposed into fatty acids such as propionic, acetic, and butyric acid. This decomposition is performed by several facultative and anaerobic bacteria such as Clostridium, Bifidobacterium, Desulphovibrio, Actinomyces, and Staphylococcus. Finally, methanogenic bacteria such as Methanobacterium, Methanobacillus, Methanococcus, and Methanosarcina digest these fatty acids, resulting in the formation of methane gas (Metcalf & Eddy, 457).

The production of methane gas is the slowest and most sensitive step of the anaerobic digestion process because it requires specific environmental conditions for the growth of methanogenic bacteria. These bacteria can only digest effectively at a pH of 6.6-7.6, and if the growth of the acid forming bacteria is excessive, there will be an overproduction of acid leading to a decrease in the pH causing many problems. (Metcalf & Eddy, 457). Also, the methanogenic bacteria have a limited temperature range for optimum performance, usually in the mesophilic range (90 - 105 °F). Often this requires pre-heating of the waste before entering the digester (Owen, 203).

Anaerobic Digesters

Utilization of Methane

Once the methane gas has been collected from the reactor, it must be cleaned and separated from other biogas constituents such as carbon dioxide, hydrogen sulfide, and excess moisture. Hydrogen sulfide is corrosive to metal piping and may damage gas engines and therefore must be removed by scrubbing the gas with an iron oxide sponge or a gas scrubber. Metal ions added to the sludge before anaerobic treatment can also reduce the hydrogen sulfide content, forming insoluble salts which are removed during digestion (ASD, 31).

Although most carbon dioxide is stripped during the removal of hydrogen sulfide, additional carbon dioxide may be removed to reduce the total volume and increase the gas value. Removal of carbon dioxide is expensive and is only economically feasible when the gas is to be sold commercially. The most common method of carbon dioxide removal is absorption through a chemical or aqueous solution, as in a scrubber (Owen, 262). After cleaning and purifying the methane gas, it can either be stored for later use or used immediately. It can either be burned by direct firing or within a gas engine (internal combustion). Digester gas can be used as fuel for hot water boilers, water pump engines, blowers, and electric generators. It can also be used to fire incinerators or burned to heat the influent sludge during pretreatment. The benefits of this process are optimized when the gas is used on site; most commonly to heat the digester influent. Any excess gas that can not be used by the treatment plan can be sold commercially (ASD, 31).

Conclusions

Thus for municipal wastewater treatment plants it is most cost effective and environmentally sound to use anaerobic digestion in the stabilization of sewage sludge. This is most efficiently done using either an anaerobic activated sludge system or a submerged media anaerobic reactor. Not only does this greatly reduce the final volume of waste to be disposed, it also creates enough energy, in the form of methane gas, to fuel the whole plant.

more info on sludge treatment

References