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Updated on September 16, 2014

Retention time, it is calculated as the volume of the digester divided by the volume added per day, and it is expressed as days.

The solids retention time represents the average time that the solids remain in the system. The solid retention time can be determined by dividing the weight of volatile solids in the system by the weight per unit time volatile solids leaving the system. The hydraulic detention time (HRT) is equal to the solids retention time in completely mixed non-recycled digester system.

Retention time has an effect upon gas production as shown in the results of Dague. There is a minimum retention time which allows the slowest growing bacteria to regenerate. Then, there is a minimum retention time required to achieve a satisfactory stabilization of the solids, which Dague shown as 10 days for sewage sludge at 350C., the gas production rate will drop and the process may fail due to a condition called wash out where the bacterial cultures decreases to the point that are no longer effective. If the retention time is greater than 10 days at 350 C., the gas production levels out and very little additional gas is produced for the additional time. Therefore, long detention times lead to low efficiency of the process.

There is a tendency to refer to gas production rates in terms of volume of gas produced per volume of digester volume. If maximum gas is desired, the ratio may be four or more volume per day. However, if the purpose of the digester is to produce and store a soil conditioner and to have some gas, as is the case in china, then the ratio may be very low and has no meaning.

Retention time along with temperature is important from the standpoint of the destruction of pathogens, if imporved health is a consideration, certain minimum values should be exceed.


Gas production is a function of the solid materials and their bio-degradability in the digester. The more concentrated the solids, the smaller the digester and the lower the cost of the system. In sewage treatment plants, efforts are made to concentrate the solids to reduce the volume and the cost of the digester.

The literature about the Indian digesters as reported by ESCAP and elsewhere implies that an optimal solids concentration of 7 to 9% of the feed should be used. However, system has been designed to use as little water as possible. For example, Jewell points out the advantage of dairy manure (10 to 13%) is that it can be added to digester directly without dilution. If the manure has stood for a few days some water may have to be added to slurry the material for introduction into the digester. In Israel the manure is scrapped from the cattle pens with bedding which is either asphalt or concrete. Marchalm reports that he has fed this manure with only small quantities of water at 16 to 18% solids content without difficulty. Batch dry digesters have operated with solids concentration at 60%. It is obvious that there is a conflit between the Israel experience on solids levels fed to digesters and the Indian recommendation.

The batch dry digesters with high solids concentrations appear to be an effective means of producing gas cheaply. Currently, efforts are underway to manage and optimize the gas production for crop residues and urban solid wastes in landfills having very high solids.


The rate at which blomass is supplied to the digester is referred to as the volumetric organic loading rate and is commonly expressed in terms of grams of volatile solids per liter of digester capacity per day (gm VS/1-day). Different loading rates can be obtained by either changing the concentration of the solids in the influent or varying the flow through the digester. In practice the solids concentrations tend to kept constant, and thus the flow rate is changed.


Two types of systems are used in digestion. The plug flow system in which no mixing takes place and the completely mixed digester. Plug flow offers the advantage in that there is no need for mixers, but some of the effluent must be re-circulated to inoculate the feed with organisms necessary to carry out the process. The advantage of the plug flow system

Mixing offers advantage in that the substrate is kept in contact with the microbes and temperature is kept uniformly districuted.

Mixing has three important effects upon the process as it:

  1. Maintains a uniformity in substrate concentration, temperature, and other environmental factors.
  2. Minimizes the formation of the scum at the surface, and
  3. Prevents the deposition of solids in the bottom.

The degree of mixing varies depending upon the feed-stock and operating conditions. Rajabapaiah reported that there was hardly any stratification inside a KVIC digester which was fed cattle manure and that the temperature profiles were within one degree throughout the digester. Hashimoto reported increases of 8% and 11% in gas production with continues mixing, over mixing only two hours per day in digester using cattle manure at 550 C and retention times of six and four days. Hashimoto's data would indicate that at longer detention times the effect of ntermediate mixing would be minimal.

Scum formation appears to be a primary fucntion of the feed. With the addition of large amounts of fibrous materials and fats, the formation of a scum layer is likely. If the organic materials are in the scum layer, it is likely that they will not be available as feed to the organism degrading the materials to gas. Thus the gas production rates in a digester with scum layers are reduced.

The reason for trying to avoid the accumulation of the solids at the bottom of the tank is the reduction of detention time. Detention time is a primary factor in gas precaution. The hydraulic detention time in an unmixed fixed dome digester was only one half the theoretical value.


Digestion progresses more rapidly at a higher temperature, therefore, it is important to get the digester feed at as high a temperature as possible and to keep the heat losses to a minimum.

Insulation to reduce the heat losses from a digester is very important. In examining some of the efforts in developing countries 54% of the total heat loss was from the cover of the Indian digesters. Thus, one can see that for cold climates the Indian digester with a floating metal cover is not a viable option. The materials used to insulate the digester vary from the use of dry agricultural residues (e.g. straw, hay, and corn stalks) to commercial poyurethane materials. The example is in Turkey the digester is built into the floor of the barn.

A number of heating techniques can be used in installation. These vary from simples solar heaters placed above the digester to heat exchangers and steam injection (bubble gun ) heating.

Solar heat can be one of two types; active or passive. Active systems heat a portion of the feed during the day and it is then digester. Passive system depend upon building a solar green-house that captures the radiant heat energy.

The membrane digester and the Chinese digester, Since the membrane digester absorbed the solar energy the temperature was higher, and it gave a larger gas production.

A share reviewed the optimization of digesters using manure as a feed stock and A share and West and A share reviewed the feasibility of using crop residues.


A Digester system needs to be located near the feed supply so the least possible amount of labour is used. As an example, in China the digester is often located near the house so that the pig pen wastes and human wastes can flow directly into the digester.

Another example of a low labor system is in Turkey where the digesters have been built under the floor in the cattle or sheep barns. This system also provides additional insulation against cold weather and makes the digester readily accessible to the supply of manure.


A slurry is discharged from the digester. The characteristics of the slurry depend upon the feed-stock, th digester conditions, and the portion of the organic matter which is converted into gas. The effluents can all be handled to the field for use as a soil conditioner or to ponds for aquatic biomass production. In some areas the slurry has been separated using vibratory screens, setting tanks, or sludge centrifuges to separate the liquid and solid portions.

The use of the slurry as a soil conditioner have been described by Ward. the important concepts to keep in mind is that the slurry should contain all the initial nutrients contained in the feed-stock. The solids will have been reduced in quantity leaving a more stabilized solid which have been partly decomposed into ammonia nitrogen. The ammonia nitrogen which is produced, while it is more available to plants, is also much easier to lose through either mis-application or drying. As an example Table based on the changes shown which occurred when diary manure was digested at mesophilic temperatures (32.50C) for 20 days.

The digester slurry often cannot be applied directly to the fields and must be stored. Some methods used to hold the materials include:

  • putting the slurry in holding ponds or tanks;
  • the sludge is placed on drying beds where the liquid containing the NH3N either drains into the soil, is lost

Change in Composition of Dairy Manure During Anaerobic Digestion (32.5 degree for 20 days)

RAW (Influent)
Slurry (effluent)
Total Solids
Reduction (%)
Volatile Solids
Reduction (%)
Total Nitrogen
Reduction (%)
Increase (%)

Prior et al. showed similar changes when digesting beef cattle manure from a concrete slab at themophillc temperatures.

Changes Composition of Beef Cattle Manure during Digestion (55 degree C for 12 days)

Total Solids
gm/1 Reduction (1%)
Volatile Solids
Reduction (1%)
Total Nitrogen
Reduction (%)
(Increase % )

to the tmosphere or is contained in the sludge; or ( C ) where vibratory screens have been used to separate the liquid and solids fractions. The effluents slurry, which is stored in open tanks or lagoons, will lose the ammonia nitrogen (NH3H) to the atmosphere, thus reducing its fertilizer value.

The solids recovery using a 60 mesh sieve and a sieve plus a sludge centrifuge. His data, which is shown, the recovery in the solids are lower than would be indicated in the liquid phase.

Recovery of Solids and nutrients by sieves and Sieve-Centrifuge (percentage of slurry contains)

Total Solids
Volatile Solids
Suspended Solids
Total Nitrogen
Organic Nitrogen

In any economic analysis, care must be taken that the credits given to the products of the digester do not exceed the least costly alternative source.

In examining the uses of the slurry, some question involve:

  • Projected characteristics of the slurry, solids, and liquid effluents;
  • Potential points and periods of use of the slurry;
  • Means and cost of storage and transport of the slurry; solids, or liquid effluents to the point use; and
  • Cost of the materials that the slurry would replace (e.g., peat, fertilizer, fish, and feed).


There is an need to build a low-cost system that will provide a gas-tight container. In China early digester used soil cement to obtain a low-cost water displacement digester. As the gas was produced the pressure would build up and would displace liquid. when reported that many of these digesters soon failed under the pressure. The designers had two choices. One was to change the materials of construction, or the second was to change the gas collection and storage system to use lower pressures. In reality, both changes were made. More cement was used in building the tank which could withstand the pressure. A gas storage bag system was developed to collect and store the gas at lower pressures. In Northern China the bags may be contained within buildings, while in southern China the bags may be outside. In Burma and others areas of the world, rubber inner tubes from auto or tractor tires have been observed being as a gas storage device. India's design uses an expensive floating metal gasholder. The height-to-diameter ratios recommended by KVIC are 2.5 to 4.1. Analyses of the systems show that a better ration from a cost standpoint was to have the height equal to the diameter. However, KVIC apparently in an empirical manner had attempted to reduce the metal used in the gas-holder. This meant that the tank was longer, and a deeper hole has to be dug. In areas with a high water table the tank could be floated out of the ground.

Not only is the metal gas holder expensive, but it is subject to corrosion and loses heat. To reduce the corrosion problems a water jacket ring is used around the tank into which the gas holder is installed. This means that the metal is not in contact with the digester contents but with water which is less corrosive. However, this additional ring raises the cost of the digester both in terms of materials and labor.

To reduce the heat loss through the floating cover, attempts have been made to use materials other than steel. These have included the use of ferrocement and plastics. While the ferrocement covers are relatively inexpensive and resistant to the loss of heat, they are difficult to transport and are subject to leakage. Plastic is resistant to corrosion and has lower heat losses but is about the same price as steel. In addition the raw materials often have to be imported which takes scarce foreign currency.

The fundamental problem with the Indian digesters is that, they are 50% or more in cost than the Chinese of the same size.

Taiwan has developed a long plug-flow digester made of red mud plastic. This has provided an inexpensive, alternative digestion system for use by farmers. The red mud digesters were being used in mainland China.


In sizing a digester two items must be taken into consideration: first, the type of digester that will be used, and second, the purpose of performance goals for which the process is used.


In sizing a digester, its primary purpose must be determined. If the primary purpose is for health, then reduction of the possible transmission of disease and temperature and detention time is very important criteria.


If the purpose of the system is to produce primarily a soil conditioner that the breakdown, stabilization, and storage of the organics and nutrients will govern the system


If the production of energy is the most important objective, the gas production should be optimized. The primary variables that effect production of the gas are:

  • Bio-biodegradability of the materials;
  • Concentration of the feed;
  • Kinetic constants; and
  • Detention time.

Temperature has a profound effect upon the digestion science the kinetic constants are influenced by increases in temperature. In developed countries the trend has been towards thermophilic digestion in addition to using more concentrated feeds and, in some cases, using pretreatment to increase the bio-degradability

In developing countries, the digesters usually operate at ambient temperatures and therefore only the bio-degradability of the feed, concentration of the feed, or detention time can be changed to optimize the process.

Therefore the critical period of the year with respect to temperature must be determined. The temperature used must be that anticipated in the digester and not the ambient air temperature. Science the anaerobic digestion process essentially stops at 100C, the detention time can be changed to optimize the process.

Therefore the critical period of the year with respect to temperature must be determined. The temperature used must be that anticipated in the digester and not the ambient air temperature. Since the anaerobic digestion process essentially stops at 100C, the digester contents must be kept warmer than this is any gas production is to be expected.

For design, gas production rates per unit volume are often used. These rates are temperature dependent. The Contois Kinetic model to describe the mathematical relationship which would allow one to predict the volumetric mthane production. The equation is :

V=(B0S0/HRT) (1-K/(HRT um-1 + K) ( 1 )

Where V=the ultimate methane rate in m3/m3 of digester

B0 is the ultimate methane yield in m3CH4

S0 is the influent volatile solids concentration in kg/m3CH4

HRT is the hydraulic retention time in days

Um is the maximum specific growth rate of the micro-organism in day-1

K is a dimensionless kinetic parameter.

Under U.S. condition the values for B0 have been determined to be :

  • Beef manure on grain ration concrete lot 0.35 + or - 0.05
  • Beef manure on grain ration dirt lot, 0.25 + or - 0.05
  • Dairy cattle, 0.20 + or - 0.05
  • Pigs, 0.50 + or - 0.05

These values indicate that because of the different feeds the ultimate gas production will vary with animal's manure. Thus pig manure is very digestible compared to the highly lignocellulosic feed given to dairy cows.

The k value are inhibitory the K value to S0 :

K = 0.8 + 0.0016e 0.06 S0 for cattle manure ( 2 )

K = 0.5 + 0.0043e 0.091 S0 for pig manure

These values indicate that overloading of the digester volatile solids ( S0 ), heavy metals, salts, or ammonia will be inhibited.

Um = 0.013 (T) - 0.129 can be determined empirically where T is the temperature between 20 and 600C. This equation was developed based upon a fit of the data between these two points and could likely be extrapolated down 150C safely.

Since the ultimate gas yields, B0 are hard to determine in the field, the approach of using the projected quantities of volatile solids that can be destroyed in a period of time at a temperature is used. In this literature, one will find references to volumetric efficiencies or these can be developed from laboratory studies of the feed-stocks.

Volumetric Efficiencies (m3 gas/m3 of digester per day )

Type of Digester Batch
Volumetric Efficiency
Detention time, day
Temperature 0C
Elephant grass
Elephant grass
Seine manure
Continuous Chinese
Manure/Night soil/Crop residues Cattle manure
9% solids

For example using a batch digester, elephant grass as a feed-stock at 270 C for 40 days detention, A volumetric efficiency of 0.35. In a continuous digester of elephant grass the volumetric efficiency was 0.83 a designer can take the data and plot it. The effects of varying detention times can then be seen at that temperature. If the digester is to be operated at another temperature, there are relationships that can approximate the gas production at these conditions.

The most common item that is overlooked in using these values is the effect of temperature.


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