Wastewater treated in activated sludge system

Wastewater treatment was treated in two continuously aerated activated sludge systems at high and low dissolved oxygen(DO), at three temperatures (10, 20 and 300C).

Wastewater treated in activated sludge systemBiochemical oxygen demand(BOD) removal was >94.8%.Meanwhile,   total N removal was significantly higher in AS2, at 64, 89 and 88 %, in AS1 at1, 24 and 46% for 10, 20 and 30⁰C, respectively.


Conventionally, biological N-removal from swine wastewater is achieved by combining autotrophic nitrification under aerobic conditions and heterotrophic denitrification under anoxic conditions.

Nitrification consists of sequential oxidation of ammonium (NH4+) to nitrite (NO2−) by ammonium-oxidizing bacteria (AOB), and nitrite to nitrate (NO3−) by nitrite-oxidizing bacteria (NOB).

Mathematical modeling is an interesting tool for studying wastewater treatment processes aiming to remove nutrients. Several activated sludge models (ASMs) available in the literature have largely been applied in the field of municipal wastewater.

However, alternative developments are necessary when dealing with highly concentrated streams. In this regard, the use of modeling for characterizing swine wastewater treatment has not often been considered.

Being proposed an extended version of the ASM1 including nitrite as intermediate of both the nitrification and denitrification processes. Alternatively, Margi and collaborators proposed a revised version of the ASM2 including nitrite as the intermediate,

Free ammonia and free nitrous acid as potential inhibitors in nitrification, pH as the state variable, and temperature as the process parameter. Both models were calibrated using data obtained from the pilot scale.

Materials and methods

Two bench-scale CSTRs operated as AS bioreactors, AS1 and AS2  were run under different aeration conditions of high DO and low DO, respectively, to assess the effect of the DO concentration on their performances.

Such continuously aerated tanks, with 0.9L for the water phase, were followed by a settler of 0.15L, where the sludge was allowed to decant and was then returned to the bioreactor.

Furthermore, excess sludge was withdrawn from the system as required. The aeration rate was manually controlled to fit DO concentrations of 1.5–2.5mg/L (typical conditions) and about 0.1mg/L (limiting conditions) forAS1andAS2, respectively.

The DO concentration in the liquid bulk was monitored by using a specific probe. The volumetric loading rate applied to the reactors was adjusted to 0.52g BOD.

During the experimental period, both bioreactors are placed in a thermostatic chamber where the temperature was sequentially adjusted to 20°C for days 1–46 (run 1), 10°C for days 47–76 (run 2), and 30°C for days 77–108 (run 3).

For calculations, stable operating conditions were assumed in days 34–46 (run 1), 63–76 (run 2), and 104–108 (run 3). The influent used to feed the AS bioreactors were prepared by mixing wastewater collected in a swine barn located at the facilities of NARO (Japan), preconditioned swine feces, NH4Cl, and KHCO3. Targeted BOD5 and NH4+-N contents of the mixture were 2000 and 500mg/L, respectively.

KHCO3 was added as a source of alkalinity to ensure a molar ratio, where t-IC is total inorganic carbon. The resulting mixture was sieved using a 0.5-mm mesh and was stored at 4°C before use.

Microorganisms i.e., bacteria are also used to treat industrial wastewater. Wastewater is collected in a pound and MO is spread in it. These microbes digest the heavy metals from the contaminated water which is then further processed and aerated to remove the concentration of contamination from the water to make it usable again for agriculture purposes.

Process description

The process takes advantage of aerobic micro-organisms that can digest organic matter in sewage, and clump together (by flocculation) 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.[1]

The general arrangement of an activated sludge process for removing carbonaceous pollution includes the following items:

  • Aeration tank where air (or oxygen) is injected in the mixed liquor.
  • Settling tank (usually referred to as “final clarifier” or “secondary settling tank”) to allow the biological flocs (the sludge blanket) to settle, thus separating the biological sludge from the clear treated water.

Treatment of nitrogenous matter or phosphate involves additional steps where the mixed liquor is left in anoxic condition (meaning that there is no residual dissolved oxygen).

Process control

The general process control method is to monitor sludge blanket level, SVI (Sludge Volume Index), MCRT (Mean Cell Residence Time), F/M (Food to Microorganism), as well as the biota of the activated sludge and the major nutrients DO (Dissolved oxygen), nitrogen, phosphate, BOD (Biochemical oxygen demand), and COD (Chemical oxygen demand).

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 in milliliters occupied by 1 gram of dry sludge solids after 30 minutes of settling in a 1000 milliliter graduated cylinder. The MCRT is the total mass (lbs) of mixed liquor suspended solids in the aerator and clarifier divided by the mass flow rate (lbs/day) 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 (lbs/day) divided by the amount (lbs) of Mixed Liquor Volatile Suspended Solids (MLVSS) under aeration.

Note: Some references use MLSS (Mixed Liquor Suspended Solids) for expedience, but MLVSS is considered more accurate for the measure of microorganisms. Again, due to expedience, COD is generally used, instead of BOD, as BOD takes five days for results.

Based on these control methods, the number of settled solids in the mixed liquor can be varied by wasting activated sludge (WAS) or returning activated sludge (RAS).

Authors:  1Zohaib Afzal, 2Faheem Shoukat

1Department of Soil Science, MNS- University of Agriculture, Multan, Punjab, Pakistan.

2Department of Entomology, MNS- University of Agriculture, Multan, Punjab, Pakistan.

Faheem Shoukat

Bachelor scholar, student of Entomology in MNS-University of Agriculture Multan

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