What is solids retention time?

There are different types of retention times depending on where the wastewater and its component stays during the treatment process. Examples include hydraulic retention time and solids retention time (SRT). SRT is discussed in this article.

What is the retention time of wastewater?

Retention time in a wastewater treatment system is the amount of time the wastewater or the components of the wastewater stays in the treatment system. Because different processes occur for wastewater treatment and each requires different retention times to get completed, the amount of time the wastewater and its components spend in different parts of the treatment system for the different processes vary from one wastewater treatment plant to another.

What is solids retention time?

In general, solids retention time (SRT) is a measure of the amount of time the solids fraction in the wastewater stays in a reactor.

How do you calculate solids retention time?

SRT is generally represented by the following equation:

Why is SRT important?

SRT affects the age of the activated sludge and the overall chemistry occurring in the wastewater treatment system, which in turn affects the quality of the treated wastewater. Specifically, SRT is important for maintaining the balance between:

• The microbial population for their growth and their decline in the activated sludge. This balance is important because it influences the rate of breakdown of organic matter including volatile solids by the microorganisms in the activated sludge and the growth of filamentous bacteria. 

• Nitrification and denitrification. Optimizing the SRT is important to ensure that the nitrification and denitrification processes have sufficient time for the conversion of ammonium to nitrate (nitrification) and then nitrate to nitrogen gas (denitrification).

• Biomass retention and aeration. Enhanced biomass retention leads to increased removal of organics and inorganics such as nitrogen and phosphorus. Maintaining the aeration level is required for this removal process to work efficiently.

Thus for maintaining and improving the quality of wastewater treatment, it is imperative to identify and manage the above.

What is the relationship between SRT and MLSS?

Mixed Liquid Suspended Solids (MLSS) is suspended solids in mixed liquor, the concentration of which is expressed in grams per liter. Mixed liquor consists of raw or settled wastewater and activated sludge.

MLSS is related to SRT because both involve activated sludge and therefore, measuring the MLSS is essential for controlling the SRT. These involve mass-balance measurement and hydraulic control techniques.

On the other hand, SRT has been shown to control MLSS. A study investigating the effects of MLSS and SRT on the performance of a wastewater treatment plant showed that both MLSS and SRT removed total nitrogen and total phosphorus at MLSS of up to 4,000 mg/L and SRT of up to 210 days. Interestingly, the study concluded that the suggested control strategy for the wastewater process control was SRT-assisted MLSS control. 

SRT, activated sludge, and sludge age

SRT can specifically apply to activated sludge, which is solid settled at the bottom of the tank and is involved in biological treatment. In this context, SRT is a measure of the average length of time the activated sludge remains in the tank and thus, SRT can also be referred to as sludge age. 

What is the retention time for activated sludge solid?

Technically by definition, SRT includes activated sludge. The retention time for activated sludge depends on several factors including the composition of the wastewater, the design of the treatment facility, and on the operational parameters for wastewater treatment. As such the retention time for activated sludge varies from one wastewater treatment facility to another. 

What is a good sludge age?

Technically, a good sludge age is sludge that stays in the tank for a specific time that has a microbial population optimized to carry out the biological processes. Sludge age depends on the factors that affect the SRT.

Factors affecting solids retention time

SRT depends on the following factors:

• The composition of the wastewater: This includes organic and inorganic chemicals besides the microbiological composition. It also includes the total amount of solids in the wastewater.

• Design and configuration of the treatment plant: The reactor type and configuration including the volume of the digester influences the aeration methods that can be applied.

• Operational parameters of the wastewater treatment plant: Temperature and pH are among these parameters that affect biological treatment processes and the chemical reactions occurring in the wastewater treatment plant.

• Biological treatment processes and chemical reactions: These reactions depend on the age of the activated sludge.

Effects of SRT on the wastewater treatment processes

SRT affects several components integral for maintaining the qualities of the effluent and the activated sludge. These effects are described as follows.

Effluent qualities: A study identified the effects of SRT on effluent polysaccharide and protein concentrations of sizes less than 1.5 microns, 0.45 microns, 30K and 3K, on the chemical oxygen demand (COD), and the biological oxygen demand (BOD). For SRT of 10 days compared to SRT of five days, the concentration of polysaccharides of the above-mentioned sizes in the effluent was higher at SRT of 10 days compared to SRT of five days. However, the concentration of effluent proteins, COD, and BOD₅ decreased at SRT of 10 days compared to SRT of five days.

Activated sludge function:  Too low of the SRT causes elevated BOD in the treated effluent, incomplete nitrification and denitrification, and elevated ammonia. On the other hand, too high of the SRT, besides causing filamentous bulking, leads to increased oxygen requirement, increased nitrate concentration, and increased total suspended solids in treated effluent.

Biological treatment: Microorganisms are involved in the biological treatment of wastewater. Specifically, they degrade some of the chemicals in the effluent. A study showed that longer SRT had a significant impact on the microbial community structure and therefore on the quality of the effluent.

Wastewater treatment is a dynamic process and several factors including the wastewater composition, the design of the facility, temperature, pH, and the microbial population in the activated sludge, are correlated with each other affecting the SRT which in turn have the above specified effects.