Introduction

Wastewater is treated using a procedure that combines biological methods of wastewater management with secondary sedimentation. Microbes (aerobically or anaerobically), algae, and fungus use the organic content in the settled wastewater in a properly built bioreactor under aerobic or anaerobic conditions (aerobically). The bacteria may proliferate and utilize the dissolved organic materials as energy in the bioreactor’s optimal bioenvironmental conditions. 

The biological oxidation of dissolved organic matter will continue as long as oxygen and food are given to the bacteria in the form of settling wastewater. Several bioenvironmental variables influence bacterial activity and biochemical reaction rates. The most critical elements are pH, Temperature, dissolved oxygen, nutrition content, and hazardous compounds. These variables may be managed inside a biological treatment system and/or a bioreactor to guarantee that microbial growth occurs in the best possible environment. 

The majority of biological methods of wastewater management work in the mesophilic temperature range, which is defined as temperatures between 20 and 40 degrees Celsius. Although the air temperature and ventilation rate may significantly influence heat loss in percolating filters, aeration tanks and percolating filters function at a wastewater temperature that ranges from 12°C to 25°C.

Types of Biological Methods

Biological methods wastewater treatment is a significant and fundamental part of the wastewater treatment process, and it processes wastewater from residential buildings, industry, and other sources. Secondary treatment is a term used to describe the process of removing impurities that remain after the first treatment. Biological methods of wastewater treatment technologies are divided into two categories based on the process:

1. Aerobics Biological Method (in presence of oxygen) 
2. Anaerobic Biological Method (in the absence of oxygen)

Aerobic process

Introduction 

A biological method of wastewater management that occurs in the presence of oxygen is known as aerobic wastewater treatment. It is the fastest and most effective method of biological waste treatment, removing up to 98 percent of organic pollutants. This procedure breaks down organic contaminants effectively and produces a cleaner water effluent than anaerobic treatment.

Aeration has been utilized to remove trace organic volatile compounds (VOCs) from water. It has also been used to oxidize iron and manganese by transferring a material, such as oxygen, from the air or a gas phase into the water in a process known as gas adsorption or oxidation. Aeration also allows dissolved gases like CO2 and H2S to escape.

Types of reactors available to treat the wastewater

Many techniques, including activated sludge, trickling filters, aerated lagoons, and oxidation ponds, are used in aerobic biological methods of wastewater Management. The most extensively used method for home and industrial wastewater is activated sludge. Types of bioreactors available for aerobic methods are

• Sequential Batch Reactors: This sort of treatment is done in batches, with all procedures taking place in the same tank at the same time. It’s a suitable option for enterprises with tiny effluents but a lot of variability in their attributes.
• Membrane Bioreactor: This sort of treatment is done in batches, with all procedures taking place in the same tank at the same time. It’s a suitable option for enterprises with tiny effluents but a lot of variability in their attributes.
• Moving Bed Biofilm Reactor (MBBR): The purifying bacterial culture adheres to specialized big surface supports (filter fill) immersed and moving in the biological reactor, creating a moving bed biofilm reactor.

Pros and Cons of Reactors

Pros: 

• Water from a wide range of sources can be treated; the only stipulation is that it be biodegradable.
• 0.4 higher yield than anaerobic (1g of organic matter for 0.4g of biomass).
• Operational simplicity.
• Produces a supernatant that has been cleared.
• For digestion, a larger number of bacteria kinds might be employed.
• The sludge respiration rate is lower.

Cons: 

• High OPEX due to continuous energy expenditure associated with aeration.
• Many parameters to control for optimum results: pH, Temperature, % Organic matter, Inlet flow, % toxic compounds (biocides).
• Higher costs during maintenance or breakdown due to maintaining the organic matter levels necessary for the microorganisms to survive.

Anaerobic process

Introduction 

This biological method of wastewater management procedure effectively treats high-strength wastewater and uses organisms that function without oxygen. It will generally treat high-strength wastewater to a level that allows it to be discharged into a municipal sewage system. Compared to aerobic treatment, the amount of sludge generated is quite little. Anaerobic treatment is a long and winding procedure with numerous phases. Anaerobic digestion is a biological method for sludge breakdown and stabilization in wastewater treatment plants. After the procedure is finished, the effluent can be treated in various ways.

Types of reactors available to treat the wastewater

Depending on the use and needs of the facility, anaerobic treatment systems can be developed and configured as single-stage or multi-stage units.

In an anaerobic treatment cycle, wastewater enters a bioreactor receptacle holding sludge, a thick semi-solid material rich in anaerobic bacteria and other microorganisms that break down the organic pollutants in the wastewater.

• Anaerobic Sludge Blanket Reactors: Wastewater passes through a layer of suspended sludge particles, where anaerobes digest the organic pollutants in the wastewater, multiplying and gathering as bigger granules that drop to the reactor tank’s bottom. Meanwhile, the treated wastewater flows upward and out of the unit, and the biogas produced is collected via collection hoods.
• Anaerobic Filter Reactors: Anaerobic microorganisms will establish themselves to create a biofilm in a reactor tank with some type of filter media. It takes many months for the biofilm to form itself and be available for treatment at maximum capacity. Wastewater passes through the filter medium during the treatment cycle, where anaerobes in the biofilm collect and act on organic material in the wastewater.

Pros and Cons of Reactors

Pros: 

• Because biomass generation is six to eight times lower than aerobic processes, sludge treatment and disposal costs are lower.
• Nutrient supplementation costs are cheaper since fewer nutrients are required due to lower biomass output.
• Anaerobic systems need less energy to operate.
• Anaerobic processes can manage larger volumetric loading rates with a small reactor volume; hence a small reactor surface is required.
• Anaerobic systems create biomass, which may be used as a fuel source.

Cons:

• Methanogenic archaea are bacteria that develop slowly.
• If the influent is rich in sulfur and/or the methanogens are unhealthy, smells might occur, resulting in exhaust containing volatile organic acids.
• pH must be maintained, and volatile fatty acids must be monitored.
• It doesn’t work well with dilute wastes or other compounds that should be processed by aerobic processes (insoluble fats, oils, grease)
• Ammonia-nitrogen is not removed.
• For most mesophilic digesters, the goal zone temperature, which is 35 to 40 degrees Celsius, must be maintained all year.