Hydraulic retention time (HRT)
The HRT is a measure of the rate of liquid flow in to and out of a reactor. Under steady state conditions, the HRT is defined as follows:
HRT = total volume of liquid in the system/volume of liquid changed per day
In a completely mixed system that employs continuous mixing, all the contents of the system have the same residence or retention time. In such a system, the detention time is governed by the replication time of the slowest growing organism of the microbial community. Below this value, the system fails from washout of the slowest growing organism that is necessary to the process. On the other hand, in systems such as the anaerobic sequencing batch reactor (ASBR), upflow anaerobic sludge blanket reactor (UASB) and induced blanket reactor (IBR), solids retention time (SRT) is decoupled through internal settling and biomass retention from HRT. The HRT can be varied independently of the SRT (Parkin and Owen, 1986; Droste, 1997).
Solids retention time (SRT)
The SRT has been recognized as a key parameter in the design and operation of anaerobic treatment processes. The SRT is the average time that a solid particle, particularly a biological particle, stays in the reactor. Under steady state conditions, the SRT is defined as follows:
SRT = mass of total biomass in the reactor/biomass wastage from the reactor per day
For successful operation of any system, a minimum SRT must be maintained to allow the working microorganisms to regenerate themselves in the system or the system eventually fails from washout of bacteria. If the slowest growing organisms have a unique role that no other species can perform, the washout causes a loss of function and a disruption or failure of the process.
The rate of regeneration or growth of a microorganism depends on various environmental factors. The factor that influences growth rate most is the temperature at which organisms are maintained. The growth rate and minimum SRT can be related to temperature using the Arrhenius equation (Dague et al., 1970). From laboratory studies of many wastes, the minimum SRT for methane formers is 3-5 days at 35 °C. All biological systems require a safety factor of 3-20 times the minimum SRT for successful operation (Lawrence and McCarty, 1969).
Droste (1997) described four methods of maintaining and increasing the amount of biological solids in anaerobic systems.
1 Separation of solids from the anaerobic digester effluent and recycling of these solids to the reactor.
2 Provision of fixed surfaces on which bacteria grow and are retained in the system.
3 Development of a dense sludge blanket to hold solids in the anaerobic system.
4 Operation of the anaerobic digester with long hydraulic detention times.
The organic loading rate represents the amount of organics that must be handled by the anaerobic system, measured in mass of organic influent to the system per unit volume per time. This parameter is used as an index of the stress imposed on the microbial population and affects the amount of total gas, methane production, COD stabilization and alkalinity.
High loading rate is one of the most important advantages of the anaerobic process compared with the aerobic process because there is no oxygen transfer limitation and no biomass thickening limitation with proper biomass immobilization (Lettinga and Hulshoff-Pol, 1991b; Speece, 1996). Maximum organic loading rate for an anaerobic reactor depends on a number of parameters, such as reactor design, wastewater characteristics, the ability of the biomass to settle and activity, etc.
In practical design, a rate of about 10 g/COD/L/day has been commonly used for high-strength wastewater in high-rate digesters. Fang and Chui (1993) showed a maximum organic loading of 100 g COD/L/day with soluble chemical oxygen demand (SCOD) removal of 90-98% in a UASB fed sucrose and milk powder.
Speece (1996) reported that the factors that control organic loading rates are:
1 Concentration of viable biosolids which can be retained in the anaerobic reactor.
2 Mass transfer between the incoming wastewater and the retained biomass.
3 Biomass proximity for metabolism of hydrogen intermediates.
4 Temperature and pH within the anaerobic reactor.
5 Level of toxicity in the wastewater.
6 Reactor configuration and presence of staging.
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