The Anaerobic Digestion Process

I. Introduction

Anaerobic digestion is the most common process for dealing with wastewater sludge containing primary sludge. Primary sludge is the solids which settle out of the wastewater in the sedimentation tanks just after the wastewater passes through the grit chambers. The settled material represents 40% to 60% of the suspended solids that exist in the wastewater. this represents 25% to 35% of the BOD in the wastewater.

BOD is a measure of the biodegradable organic matter in the wastewater. It is determined by the amount of oxygen required to metabolize the organic matter in the water.

Secondary sludge from the clarifier is also sent to the digester. Secondary sludge is generated when the over flow from the settling tanks goes into the aeration chambers and the aerobic bacteria convert the dissolved organics into carbon dioxide, water and solids. The solids settle out in the clarifier.

Anaerobic digestion is preferred to reduce the high organic loading of primary sludge because of the rapid growth of the biomass that would ensue if the sludge were treated aerobically. Anaerobic decomposition creates considerably less biomass than the aerobic process. Anaerobic digestion converts as much of the sludge as possible to end products, such as, liquid and gases while producing as little residual biomass as possible.

The liquids, in the form of supernatant, from the digester are sent back through the plant for further treatment.

Therefore, in a sewage treatment facility the aerobic and anaerobic processes work together to achieve BOD removal of up to 95%.

An anaerobic sludge digester is designed to encourage the growth of anaerobic bacteria, particularly the methane producing bacteria that decreases organic solids by reducing them to soluble substances and gases, mostly carbon dioxide and methane.

There are three basic stages in anaerobic digestion. The first stage is production of carbon dioxide and organic acids from fermentation. The second stage is metabolizing of organic acids to hydrogen, carbon dioxide and other organic acids. The third stage utilizes the products of the preceeding stages to produce methane from carbon dioxide, hydrogen and acetic acid.

The sludge that remains is relatively stable and inert. From 50% to 60% of the organics are metabolized with less than 10% converted to biomass.

The anaerobic process is made up of two basic types of bacteria. The acid formers and the methane formers. The acid formers are facultative and anaerobic bacteria and include organisms that solubilize organic solids through hydrolysis. Soluble products are then fermented to acids and alcohols of low molecular weight. The methane formers are strict anaerobics that convert acids and alcohol along with hydrogen and carbon dioxide to methane.

Stability of the anaerobic process is difficult to maintain because a balance favorable to several microbial populations is necessary. The methane producers are the most sensitive to conditions. They may be affected by change in the pH of the digesting sludge. Each species is limited to the use of a few compounds, mostly alcohols and organic acids. The rugged nature of the acid formers and the sensitive nature of the methane formers creates a bio system that is easily upset.

II. Description of the Anaerobic Digestion Process

The purpose of the anaerobic process is to convert sludge to end products of liquid and gases while producing as little biomass as possible. The process is much more economical than aerobic digestion.

It was originally thought that anaerobic digestion was accomplished in three stages: (1) hydrolysis of insoluble polymers, (2) fermentation of monomeric breakdown products and (3) generation of methane. Research has found that there is a step between fermentation and methane generation, the fermentation of acetate and hydrogen from volatile fatty acids. These bacteria are necessary to the process and are even more sensitive than the actual methane formers.

The process has now been described by the following four steps:

Hydrolysis: large polymers are broken down by enzymes.
Fermentation: Acidogenic fermentations are most important, acetate is the main end product. Volatile fatty acids are also produced along with carbon dioxide and hydrogen.
Acetogenesis: Breakdown of volatile acids to acetate and hydrogen.
Methanogenesis: Acetate, formaldehyde, hydrogen and carbon dioxide are converted to methane and water.

The stability of the anaerobic process is very fragile. The balance between several microbial populations must be maintained. The hydrolysis and fermentation phases have the most robust organisms. They have the broadest environment range in which they thrive. They react quickly to increased food availability. Thereby, increasing the amounts of their products. The volatile fatty acid concentration rises very quickly. This is kept in check by the buffering action of the system provided by carbon dioxide in the form of biocarbonate alkalinity. The pH range is, therefore maintained under normal circumstances. However, during shock loading the acid concentration can overcome the buffering action and raise the pH out of the narrow acceptable limits of the acetogens and the methogens. When this happens methane production stops and the acid levels rise to the tolerance level of the acid formers. At this point the system fails.

Temperature is also a critical element. Sudden changes in temperature adversely affect the methane producers.

Several substances are toxic to the system such as heavy metals, chlorinated compounds and detergents. Pretreatment would be necessary for a wastewater high in these.

When operating properly, the digester receives sludge, primary and secondary, from the other treatment processes. The sludge is then held in the tank for 10 to 90 days depending on the system. The sludge goes into the digester, methane, carbon dioxide and traces of hydrogen sulfide go out the gas outlet, supernatant, from the water generated by the process and the water in the sludge, is drawn off as necessary and sent back through the plant and stabilized sludge is pulled off the bottom to go to the drying beds.

The preceeding drawing shows a standard rate anaerobic digester. Reactors for anaerobic digesters consist of closed tanks with air tight covers. Treatment plants processing less than 4000 cubic meters/day of wastewater often use standard rate digestion for economic reasons or simplicity of operation. Sludge separates in the reactor as shown, although some mixing occurs in the zone of active digestion and in the supernatant because of withdrawal and return of heated sludge. Sludge is fed to the reactor on an intermittent basis and the supernatant is withdrawn and returned to the secondary treatment unit. The digested sludge accumulates in the bottom to await removal to sludge disposal facilities.

High rate digesters are more efficient and often require less volume than single stage digesters. In the first stage the sludge is mechanically mixed to ensure better contact between the organics and the bacteria. The unit is heated to increase the metabolic rate of the microorganisms, thus speeding up the digestion process. In the second stage the sludge is allowed to stratify and separate into layers. Little gas is generated in the second stage. The second stage has a floating cap and is equipped for gas recovery. The second stage is not heated because gas production doesn't occur in this stage. The supernatant, scum and digested sludge are drawn out of this unit.

III. Operational Limits and Failure Criteria

As has been previously stated the methanogens are temperature and pH sensitive. They operate in a pH range of 6.5 to 7.5. Movement to either side of this range quickly affects their metabolic rates and slows or stops methane production. Also, the optimum temperature is 95°F. Methane production drops off either side of this temperature. The methane formers are the bottleneck in the system and must be catered to. Whenever methane production drops the volatile fatty acids begin to build up quickly due to the robust nature of the acid forming bacteria. Any shift adverse to the methane formers increase acids which in turn reduces methane formers.

Pending failure of the anaerobic process is evidenced by decreased gas production, and increase in volatile fatty acids and a drop in pH when acids exceed the buffering capacity of the ammonium biocarbonate in solution.

Failure is caused by:

Significant increase in organic loading (shock loading).
Sharp decrease in digesting sludge volume (when sludge withdrawn).
Sudden increase in operating temperature.
Accumulation of a toxic or inhibiting substance.
Removal of too much of the supernatant thus reducing the number of bacteria available to metabolize.

**Conditions for Sludge Digestion**
Temperature	Optimum General Range	98°F 85-95°F
pH	Optimum General Range	7.0-7.1 6.7-7.4
Gas production	Per pound volatile solids added Per pound volatile solids destroyed	8-12 cu. ft. 16-18 cu. ft.
Gas Composition	Methane Carbon Dioxide Hydrogen Sulfide	65-69% 31-35% Trace
Volatile Acid Concentration As Ascetic Acid	Normal Maximum	200-800 mg/L 2000 mg/L
Alkalinity Concentration As Calcium Carbonate	Normal	2000-3500 mg/L

**Loading and Detention Times**
	Single Stage Digester	High Rate Digeseter (First Stage)
Loading (1 lb.cu.ft./day of VS	.02 - .05	.1 - .2
Detention time (days)	30 - 90	10 - 15
Capacity of Digester (cu.ft./pop. equivalent)
Primary	2 - 1	.4 - .6
Primary and Secondary	4 - 6	.7 - 1.5
Volatile Solids Reduction	50 - 70	50

1.0 lb/cu.ft./day = 16.0 kg/cu. meter/day

IV. Operational Guidelines and Testing

Anaerobic systems are having a resurgence in use because of a better understanding of process failure and ways to avoid it. The lower operational costs due to the avoidance of aeration and the use of methane as an energy source has helped increase popularity. Also, research has found that anaerobic systems have the ability to tolerate much higher loadings than previously thought possible.

The following guidelines are used to prevent failure:

Maintain constant loading rate by using storage capacity of the settling tanks and clarifier to smooth out the raw sludge input.

Make sure retention time is sufficient to accommodate the slowest growing organisms by not drawing off excess amounts of digested sludge.

Maintain temperature at the optimum without allowing it to increase and kill the bacteria. Allowing sludge and supernatant capacity to drop without checking heaters could cause this problem.

At all but the highest organic loading microbial growth will proceed at submaximal rate. This is difficult with such adverse system.

Make sure gas composition is in the proper range.

Maintain pH balance. Use lime if necessary.

The following records should be kept. (Testing)

Plot the daily gas production per unit raw sludge feed. This can be done taking the volatile solids of the raw sludge and the metered gas production.

Plot the percentage make up of the gases. Methane, carbon dioxide and hydrogen sulfide to ensure that digestion is proceeding properly. This should be done several times a day using a gas analyzer such as a Rankine Meter.

Plot the pH of the digester sludge to maintain the proper range. Because of the narrow band (6.7 to 7.4) Litmus paper shoudl be used. Lime may be used to prevent acidic build up.

Plot the percentage of volatile fatty acids in the digester. Natural buffering will prevent an imbalance from showing up immediately.

Plot the temperature of the sludge. Use a continuously recording thermometer to indicate trends up or down.

Plot the organic make up of digested sludge to make sure that the process is working properly. This can be done by drying the sludge. Weighing a sample and placing it in the oven at 500 degrees, comparing the residue to the original.

V. Procedure for Determining Factors Critical for Maximum Digester Efficiency

Maximization of the anaerobic process is very difficult due to the varied types of microorganisms involved. It is necessary to test the entire process with a known food to determine exactly what by products are being formed and at what rate. This will require the construction of a model digester. For simplicity, a single stage digester should be used. The information available indicates a better volatile solids removal from the slower single stage model. The model should be at least 15 gallons in capacity, including gas storage, to allow the use of volumes and weights that are easy to measure and will reduce error. The system must be set up to resemble an operating unit to the greatest extent possible.

The first step is to define the process in known terms. A control experiment must be run with all parameters set at the known optimum conditions to set a base line.

Temperature	98°F (35°C)
pH	7.0
Volatile Acid	500 mg/L
Alkalinity	2750 mg/L
Loading (VS)	.035 lb. cu.ft./day

These parameters will be the mid point for all experiments. To ensure a strong culture of bacteria a good food source shall be used such as flour. This will give a very pure food source that is easy to handle and analyze. The food will be mixed at .035 pounds per cubic foot of liquid or .56 ounces per cubic foot. This equates to .075 ounces per gallon of distilled water or .56 grams per liter. This is the mid point leveling range for a single stage digester. Because of the rich pure food source a shorter digestion period can be expected with the lab model. Also the by products should be more pure. No toxic materials will be inadvertently added to the process.

The digester will be set up with a continuous flow automatic system. The food will be added at a predetermined rate, supernatant will have a flow retarder that allows liquid to discharge at a certain level, gas storage will be set up to take off gas into a graduated cylinder to allow measurements. A fluid deplacement method of gas storage would be the most accurate and easy to read.

The initial run will be at the afore mentioned parameters. Fresh cow manure will be used to start the reaction. The reactor will be filled with a mixture of cow manure and the prepared food. The volume of prepared food will be measured (.56 grams of flour per liter of distilled water). The digester will be allowed to sit until the reaction is underway. At that time all the automatic systems will be engaged. The temperature of the digester will be maintained at 98°F at all times.

Daily measurements (from day seed and food are put in digester)

Temperature - to .1 degrees

Volatile fatty acid concentration

Alkalinity concentration

Gas production volume per day
Gas production per gram of flour added (expected range .017 to .026)
Gas production per gram of flour converted (expected range .035 to .040)

Gas composition - methane - carbon dioxide, hydrogen sulfide

pH of supernatant in digester

pH of sludge in digester

pH of supernatant affluent

pH of sludge - out

Volatile Solids in supernatant

Volatile Solids in sludge removed

Model digester will be run until a sufficient set of baseline data is accumulated.

By measuring the gas production and gas composition the optimum feed range in leters per day can be determined.

The easiest condition to control for a digester is the temperature. An interesting fact about the start up of a digester is that the bacteria needed comes from cow manure. The body temperature of cattle is from 100 to 101°F. The texts agree that 98°F is an optimum temperature and that normal ranges are from 85°F to 95°F. The reasons for this may have nothing to do with the optimum conditions for bacterial growth. Cold digesters work, but have digestion times of over 90 days.

The surest method of determining the strength of the reaction in the digester is the gas production. Temperature will be increased 1°F per week and all measurements recorded until gas production begins to drop. It will be necessary to check to be sure that the biosystem is not starving. The total gas production must be checked against total amount of food added. Therefore food must be increased to ensure proper digestion if needed.

Temperature should be the easiest factor to determine because it controls the metabolic rate of the biosystem. Therefore the peak temperature found in the variable temperature experiment should be true for all different loadings and foods.

After completing the time necessary to find the optimum temperature the digester will be brought back to the optimum temperature slowly and stay there. All measurements will be made to ensure that the changes in the process are noted during all changes. The next step is to begin to increase the food concentration to determine the optimum loading rate. It has been determined that all but the highest organic loading microbial growth will proceed at submaximal rates.

Loading will begin at the level achieved at the optimum temperature, if that is different from the original setting, and be increased at a rate of 10% per week. This will allow sufficient time for the process to adjust to the increased food rate.

All measurements will be continued and the loading will increase to system failure.