What is Retention time in wastewater treatment and its type?
Retention time refers to the amount of time that a volume of wastewater is stored within the pumping installation, pump sump, pipeline, detention basin, and other similar structures. To calculate retention time, divide the holding volume by the discharge flow from the holding structure.
Long and short wastewater retention times
It can be advantageous in some situations while being detrimental in others.
Solid particles from wastewater may settle during a long retention time, if the flow or turbulence is not strong enough to keep the particles suspended. Furthermore, extended retention times allow biological and chemical processes to occur. In some situations, such as the oxidation of hazardous substances, these processes can be useful, but in others, they can be destructive.
Too short retention times, on the other hand, may imply that desired biological or chemical processes, such as organic matter decomposition do not have time to occur, but may be intended in other cases.
Types of Retention Time
There are two types of retention times in an anaerobic digester: solid retention time (SRT) and hydraulic retention time (HRT).
The SRT and HRT for a suspended-growth anaerobic digester with no recycling are the same. If the digester includes solids recycling, the SRT and HRT may differ dramatically.
· Time Required for Solids Retention
The solids retention time (SRT) is the amount of time spent in a treatment unit, by the solid fraction of wastewater. It is the amount of solids kept in the reactor divided by the amount of solids emitted by the reactor each day:
SRT = V *Cd / Qout* Cout
Where V is the digester volume, Cd is the solids concentration, Qout is the daily volume wasted, and Cout is the effluent solids concentration. The SRT exceeds the HRT when retaining biomass reactors, e.g., anaerobic biogas settlers, septic tanks.
The solids retention time, or SRT, controls bacterial concentrations throughout the treatment system. The higher the SRT, the more bacteria there are in the reactor. High SRT levels are beneficial to anaerobic digesters. High SRT values maximize removal capability, reduce digester volume requirements, and provide buffering capacity in wastewaters and sludges, to protect against shock loadings and hazardous chemicals.
· Hydraulic Retention Time
In an anaerobic digester, the HRT regulates the conversion of volatile solids to gaseous products. The final destination of the digested sludge determines the design of the HRT. The HRT may be relatively high or low depending on, whether the digested sludge is to be land applied or burned.
The HRT regulates the time for absorption and biodegradation in water by measuring, how long a soluble chemical stays in the bioreactor (measured as the volume of the aeration tank divided by the influent flow rate).
HRT values influence the rate and extent of methane generation. Thus, among all the operational conditions within an anaerobic digester, such as temperature, solids concentration, and volatile solids content of the feed sludge.
HRT plays an important role in the production of hydrogen and methane:
1. When the HRT is very low, the reactor is washed out, which means that all of the active microorganisms leave the reactor.
2. A good HRT generates a large amount of hydrogen and methane. This characteristic is linked to the distinct and different growth rates of hydrogen- and methane-producing bacteria.
3. Low HRT promotes methanogen washout, ensuring the continued existence of hydrogen producers. As a result, low HRT and a slightly acidic pH (6.0-6.5) are ideal conditions for hydrogen production.
4. Conversely, as HRT increases, the hydrogen fermentation pattern tends to shift to methanogenic.
How can we help?
When designing a sewer system, the retention time in pipelines should be estimated and the implications considered.
Netsol Water ensures better wastewater network system design, assisting in the optimization of retention time in wastewater treatment. If you have any questions about wastewater treatment plants, or if you would like to discuss your specific needs, please contact the Netsol Water team right away. Our team is an expert in water and wastewater treatment and can help you tailor a solution, to your specific requirements.