1] Basic factors 2] Environmental factors
1] Basic Factors :
These factors includes
a) Bacteria
b) Food
c) Contact
d) Time
a) Bacteria :
Depending upon the end products produced by the bacteria they are classified as
i) Acid formers ii) Methane formers or Methanogens
The acid formers are less sensitive to environmental changes and fast growing. The optimum pH range for satisfactory operation of the acid formers is found to be 4.5 to 6.5.
Methane formers are not only specific in the end product but are also specific about the substrate for utilization. These bacteria are pH sensitive and the optimum pH for their operation is found to be in between 6.5 to 7.6. As compared to the acid formers they are less in number and the reason for their low population is related to the end product they produce methane gas. Because methane is a high energy end product subjected to further combustion to carbon dioxide and water, the energy is lost for the bacteria and is not available for growth.
The four principle characteristics of methane forming bacteria are.
i) pH sensitive
ii) Low numbers
iii) Strictly anaerobic
iv) Slow growing
b) Food :
The organic matter to be stabilized is food for bacteria. In anaerobic digestion generally food consists of complex organic materials. The control over the organic characteristics of waste is very difficult. However two factors related to the food can be controlled are solids concentration and frequency of feeding.
c) Contact :
Stabilization cannot occur without actual contact of the bacteria with the food. This contact can take place in several ways but the most effective is mixing. Mixing can be achieved artificially by mechanical mixers or by natural means. Nature mixing occurs primarily by gas production in the digester. In case of mechanical mixing the contents may be stirred continuously or intermittently.Better COD removal efficiency and increased gas production can be achieved by intermittent mixing recirculation of liquid by manual or mechanical means can also be adopted for mixing.
d] Time :
Two main factor may be considered in this aspect namely hydraulic retention time and the solids retention time. The solids retention time reflects the average time for which the biological solids remains in the digester and can be defined as Suspended solids in the system to the Suspended solids removed per day
Use of suspended solids as an indication of the microorganism is adequate in the above formulation since the average retention time of the microorganisms will closely approximate the average retention time of the mixed suspended solids in the system.
SRT depends on the degree of sludge retention achieved and HRT. The SRT/HRT ratio, therefore, directly implies the efficiency of a treatment system. Higher the ratio, more efficient and economic the system will be because maximum SRT gives efficient treatment and minimal HRT reflects the smaller size of reactor which means economy of the process .
2] Environmental Factor :
These factors include
a) Temperature
b) pH
c) Toxics
a) Temperature :
The time required for the stabilization of organic matter depends on the temperature of the digestion. The rate of food stabilization increases and decreases with temperature within certain limits.
b] pH :
The pH of the contents of a digester depends on the relationship between the volatile acid, alkalinity and percentage of carbon dioxide in the digester gas. Many reporters have indicated that the optimum pH for the digestion of organic waste is in the range 6.8 to 7.2 with the limit of the range for operation without significant inhibition being 6.5 to 7.6.
c) Toxics :
Materials which have inhibitory effects on digestion if their concentration become to high include the alkali and alkaline earth cations such as sodium, potassium, calcium and magnesium heavy metals such as copper, nickel and zinc; ammonia and ammonium ion; sulphides and some organic compounds.e.