Activated sludge
The activated sludge 'process' is a type of biological wastewater treatment process for treating sewage or industrial wastewaters using aeration and a biological floc composed of bacteria and protozoa. It is one of several biological wastewater treatment alternatives in secondary treatment, which deals with the removal of biodegradable organic matter and suspended solids. It uses air and microorganisms to biologically oxidize organic pollutants, producing a waste sludge containing the oxidized material.
The activated sludge process for removing carbonaceous pollution begins with an aeration tank where air is injected into the waste water. This is followed by a settling tank to allow the biological flocs to settle, thus separating the biological sludge from the clear treated water. Part of the waste sludge is recycled to the aeration tank and the remaining waste sludge is removed for further treatment and ultimate disposal.
Plant types include package plants, oxidation ditch, deep shaft/vertical treatment, surface-aerated basins, and sequencing batch reactors. Aeration methods include diffused aeration, surface aerators or, rarely, pure oxygen aeration.
Sludge bulking can occur which makes activated sludge difficult to settle and frequently has an adverse impact on final effluent quality. Treating sludge bulking and managing the plant to avoid a recurrence requires skilled management and may require full-time staffing of a works to allow immediate intervention. A new development of the activated sludge process is the Nereda process which produces a granular sludge that settles very well.
Purpose
The activated sludge process is a biological process used to oxidise carbonaceous biological matter, oxidising nitrogenous matter in biological matter, and removing nutrients.Process description
The process takes advantage of aerobic micro-organisms that can digest organic matter in sewage, and clump together by flocculation entrapping fine particulate matter as they do so. It thereby produces a liquid that is relatively free from suspended solids and organic material, and flocculated particles that will readily settle out and can be removed.The general arrangement of an activated sludge process for removing carbonaceous pollution includes the following items:
- Aeration tank where air is injected in the mixed liquor.
- Settling tank to allow the biological flocs to settle, thus separating the biological sludge from the clear treated water.
Treatment of nitrogenous or phosphorous matter comprises the addition of an anoxic compartment inside the aeration tank in order to perform the nitrification-denitrification process more efficiently. First, ammonia is oxidized to nitrite, which is then converted into nitrate in aerobic conditions. Facultative bacteria then reduce the nitrate to nitrogen gas in anoxic conditions. Moreover, the organisms used for the phosphorus uptake are more efficient under anoxic conditions. These microorganisms accumulate large amounts of phosphates in their cells and are settled in the secondary clarifier. The settled sludge is either disposed of as waste activated sludge or reused in the aeration tank as return activated sludge. Some sludge must always be returned to the aeration tanks to maintain an adequate population of organisms.
The yield of PAOs is reduced between 70 and 80% under aerobic conditions. Even though the phosphorus can be removed upstream of the aeration tank by chemical precipitation, the biological phosphorus removal is more economic due to the saving of chemicals.
Bioreactor and final clarifier
The process involves air or oxygen being introduced into a mixture of screened, and primary treated sewage or industrial wastewater combined with organisms to develop a biological floc which reduces the organic content of the sewage. This material, which in healthy sludge is a brown floc, is largely composed of Saprotrophic bacteria but also has an important protozoan flora component mainly composed of amoebae, Spirotrichs, Peritrichs including Vorticellids and a range of other filter-feeding species. Other important constituents include motile and sedentary Rotifers. In poorly managed activated sludge, a range of mucilaginous filamentous bacteria can develop - including Sphaerotilus natans, Gordonia, and other microorganisms - which produces a sludge that is difficult to settle and can result in the sludge blanket decanting over the weirs in the settlement tank to severely contaminate the final effluent quality. This material is often described as sewage fungus but true fungal communities are relatively uncommon.The combination of wastewater and biological mass is commonly known as mixed liquor. In all activated sludge plants, once the wastewater has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated supernatant is run off to undergo further treatment before discharge. Part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new wastewater entering the tank. This fraction of the floc is called return activated sludge.
The space required for a sewage treatment plant can be reduced by using a membrane bioreactor to remove some wastewater from the mixed liquor prior to treatment. This results in a more concentrated waste product that can then be treated using the activated sludge process.
Many sewage treatment plants use axial flow pumps to transfer nitrified mixed liquor from the aeration zone to the anoxic zone for denitrification. These pumps are often referred to as internal mixed liquor recycle pumps. The raw sewage, the RAS, and the nitrified mixed liquor are mixed by submersible mixers in the anoxic zones in order to achieve denitrification.
Sludge production
Activated sludge is also the name given to the active biological material produced by activated sludge plants. Excess sludge is called "surplus activated sludge" or "waste activated sludge" and is removed from the treatment process to keep "food to biomass" ratio in balance. This sewage sludge is usually mixed with primary sludge from the primary clarifiers and undergoes further sludge treatment for example by anaerobic digestion, followed by thickening, dewatering, composting and land application.The amount of sewage sludge produced from the activated sludge process is directly proportional to the amount of wastewater treated. The total sludge production consists of the sum of primary sludge from the primary sedimentation tanks as well as waste activated sludge from the bioreactors. The activated sludge process produces about of waste activated sludge. is regarded as being typical. In addition, about of primary sludge is produced in the primary sedimentation tanks which most - but not all - of the activated sludge process configurations use.
Process control
The general process control method is to monitor sludge blanket level, SVI, MCRT, F/M, as well as the biota of the activated sludge and the major nutrients DO, nitrogen, phosphate, BOD, and COD. In the reactor/aerator and clarifier system, the sludge blanket is measured from the bottom of the clarifier to the level of settled solids in the clarifier's water column; this, in large plants, can be done up to three times a day.The SVI is the volume of settled sludge occupied by a given mass of dry sludge solids. It is calculated by dividing the volume of settled sludge in a mixed liquor sample, measured in milliliters per liter of sample, by the MLSS, measured in grams per liter. The MCRT is the total mass of mixed liquor suspended solids in the aerator and clarifier divided by the mass flow rate of mixed liquor suspended solids leaving as WAS and final effluent. The F/M is the ratio of food fed to the microorganisms each day to the mass of microorganisms held under aeration. Specifically, it is the amount of BOD fed to the aerator divided by the amount of Mixed Liquor Volatile Suspended Solids under aeration. Note: Some references use MLSS for expedience, but MLVSS is considered more accurate for the measure of microorganisms. Again, due to expedience, COD is generally used, in lieu of BOD, as BOD takes five days for results.
To ensure good bacterial settlement and to avoid sedimentation problems caused by filamentous bacteria, plants using atmospheric air as an oxygen source should maintain a dissolved oxygen level of about 2 mg/L in the aeration tank. In pure oxygen systems, DO levels are usually in the range of 4 to 10 mg/L. Operators should monitor the tank for low DO bacteria, such as S. natans, type 1701 and H. hydrossis, which indicate low DO conditions by elevated effluent turbidity and dark activated sludge with foul odours. Many plants have on-line monitoring equipment that continuously measures and records DO levels at specific points within the aeration tank. These on-line analysers send data to the SCADA system and allow automatic control of the aeration system to maintain a predetermined DO level. Whether generated automatically or taken manually, regular monitoring is necessary to favour organisms that settle well rather than filaments. However, operating the aeration system involves finding a balance between sufficient oxygen for proper treatment and the energy cost, which represents approximately 90% of the total treatment cost.
Based on these control methods, the amount of settled solids in the mixed liquor can be varied by wasting activated sludge or returning activated sludge.
The returning activated sludge is designed to recycle a portion of the activated sludge from the secondary clarifier back to the aeration tank. It usually includes a pump that draws the portion back.
The RAS line is designed considering the potential for clogging, settling, and other relatable issues that manage to impact the flow of the activated sludge back to the aeration tank. This line must handle the required flow of the plant and has to be designed to minimize the risk of solids settling or accumulating.