How to get Treatment efficiency of Aerated ASP?
All dissolved and fine colloidal organic materials cannot be removed by the initial treatment!
As a result, eliminating the remaining components will necessitate a biological treatment.
Microorganisms that feed on organic pollutants in the water/wastewater are at the heart of all biological activities. Bacteria, algae, fungus, protozoa, and nematodes are used in the biological treatment to degrade the organic material into inorganic forms, removing the BOD (Biochemical Oxygen Demand).
Aerobic processes, anaerobic processes, and pond processes are the three types of biological treatments. Suspended growth systems and attached growth systems are the further two types.
The activated sludge technique is one of the most used biological wastewater treatments.
Description of the process of activated sludge
Bacteria are continuously combined with the wastewater in the activated sludge treatment and thus, digest the organic pollutants. The wastewater is injected with air to meet the microorganisms' oxygen needs and to keep the solids suspended and in contact with the bacterium. Bacteria clump together as they multiply, forming bigger particles that settle to the bottom of the tank. This is known as 'activated sludge.' Some of the sludge is reintroduced to the wastewater to help keep the tank's bacteria levels in check.
The procedure can be designed in a variety of ways!
Every set-up has its own set of benefits as well as drawbacks. As a result, when constructing the reactor, the configuration that is most suited to the specific process conditions should be chosen.
The following are the most typical configurations:
1: Configuration that is simple and traditional;
2: Reactors in batches;
3: Mixing everything together;
4: Aeration for a longer time.
What are the calculations and control factors?
Knowledge of five biological and physical variables that impact the effectiveness of the activated process improves the performance of an activated sludge plant. These elements include:
1: The aeration tank loaded with organic and hydraulic material;
2: Dissolved oxygen (DO);
3: The rate at which biosolids decompose;
4: Return Activated Sludge Rate;
5: Characteristics of solids settling and compaction.
1: The aeration tank loaded with organic and hydraulic materials
The number of pounds of biochemical oxygen demand (BOD) that enter the activated sludge process each day is referred to as organic loading. The million gallons per day (MGD) flow to a wastewater treatment process is referred to as hydraulic loading.
The number of microorganisms or their quantity
MLVSS (mixed liquor volatile suspended solids) is the amount of microorganisms in an activated sludge system described as the organic or volatile suspended solids in an aeration tank's mixed liquor. This volatile component is used to determine the number of microorganisms present.
The following formula is used to determine the number of microorganisms accessible for treatment:
MLVSS (lbs.) = (Aeration, mg) x (MLVSS, mg/L) x (Conv, 8.34)
2: Dissolved oxygen (DO)
The molecular (atmospheric) oxygen dissolved in water or wastewater is referred to as dissolved oxygen in the aeration tank.
The goal of keeping DO in the aeration tank is to ensure that the biology (micro-organisms) in the aeration process stays alive and removes organic debris as efficiently as possible.
The following are two reasons to keep the DO level in the aeration tank between 1.0 and 2.0 milligrams per litre (mg/L):
1: Lowering the DO improves the rate of oxygen transfer (which is proportional to the difference between real and saturation dissolved oxygen of 9 to 10 mg/L, temperature dependent);
2: Because the energy required for oxygen transfer decreases as the gap between saturation and operating increases, a DO of 1.0 mg/L is preferable to a DO of 2.0 mg/L.
I: Ratio of food to microorganisms
The amount of food delivered to bacteria in an aeration tank is known as the food-to-microorganism (F/M) ratio.
One of the key controls utilized in activated sludge reactors is the F: M ratio. This allows the operator to keep the amount of food available in balance with the amount of bacteria in the tanks. The wastewater BOD represents the food accessible to bacteria.
While the best treatment may not occur at the same F:M ratio in each plant, the range of traditional activated sludge plants is typically 0.20 to 0.50.
The F:M ratio in activated sludge facilities that operate in the extended aeration mode is normally in the 0.045 to 0.20 range.
The F:M ratio is modified by adjusting the amount of MLVSS in the secondary system, because the operator normally has no control over the BOD entering the wastewater treatment plant.
If more biomass is required, i.e., higher MLVSS, the amount of biomass wasted must be reduced, however, if less biomass is required, i.e., lower MLVSS, the wasting rate must be increased until the required biomass is reached.
There are two things to keep in mind when implementing operational changes:
1: Because biological systems are sluggish to respond to these types of control adjustments, give the system time to adjust before making another change;
2: Recognize that consistency is typically the key to a successful operation; as a result, try using a moving average to compute the BOD and only make adjustments as needed.
ii: Time spent in a cell or cell residence time
The period of time a particle has been kept in the activated sludge process is referred to as cell residence time (CRT) or sludge age (SA).
3: The rate at which biosolids decompose
The excess development of microorganisms that must be eliminated from the process to preserve the biological system's stability is referred to as waste activated sludge (WAS). Pumping a part of the return sludge to the solids handling facility accomplishes this.
Choosing a target CRT or SA is the first step in determining a waste rate. For example, the CRT for a traditional activated sludge plant should be at least seven days, while the CRT for an extended air plant should be at least 19 days.
4: Return Activated sludge Rate
The settling activated sludge accumulated in the secondary clarifier before it is returned to the aeration basin to mix with incoming primary treated effluent is referred to as return activated sludge (RAS).
When the pounds of solids extracted from the secondary clarifier (RAS) equals the pounds of material entering the secondary clarifier (MLSS), the RAS flow rate is calculated using the poundage formula as follows:
Rq = Q x MLSS
‘Rq’ is the computed RAS (in milligrams per litre), ‘Q’ is the influent flow rate (in milligrams per litre), and ‘MLSS’ is the mixed liquid suspended solids concentration in milligrams per litre.
5: Characteristics of solids settling and compaction
The sludge volume index (SVI) and the sludge density index can be used to determine solids compaction characteristics (SDI).
>SVI:SVI is the volume in milliliters occupied by one gram of activated sludge after 30 minutes of settlement. Sludge volume in milliliters to MLSS concentration in grams per litre is calculated as follows:
SVI (Standard Variable Index) = MLS settled
MLSS (milligrams per litre) / 1,000
>SDI:After 30 minutes of gravity settling, the SVI is defined as the grams of activated sludge that fills a volume of 100 ml. It is defined as SDI (Standard Deviation Index). The SDI is calculated as follows:
SDI = MLSS, mg/L / 1,000
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