Introduction to Biological Treatment | Environmental Engineering - Civil Engineering (CE) PDF Download

BIOLOGICAL TREATMENT

The physical processes that make up primary treatment are augmented with processes that involve the microbial oxidation of wastes. Such processes are biological or secondary processes that utilize microorganisms to oxidize the organics present in the waste. Main objectives of biological treatment are: 

• To oxidize dissolved and particulate biodegradable constituents into non- polluting end products. 
• To remove or transform nutrients such as nitrogen and phosphorous. 
• To capture non- settleable and suspended solids into a biofilm. 
• To remove specific trace organic compounds.

Biological treatment is basically divided into two main categories: a) aerobic processes, and b) anaerobic processes. Aerobic means in the presence of air (oxygen) while anaerobic means in the absence of air (oxygen). These two terms are directly related to the type of bacteria or microorganisms that are involved in the degradation of organic impurities in a given wastewater and the operating conditions of the bioreactor.

Aerobic Processes: Aerobic treatment processes take place in the presence of air and utilize those microorganisms (also called aerobes), which use molecular/free oxygen to assimilate organic impurities i.e. convert them in to carbon dioxide, water and biomass.

Anaerobic Processes: The anaerobic treatment processes take place in the absence of air (molecular/free oxygen) by those microorganisms (also called anaerobes) which do not require air (molecular/free oxygen) to assimilate organic impurities. The final products of organic assimilation in anaerobic treatment are methane and carbon dioxide
 gas and biomass .

Figure 4.1.1.  Mechanism of aerobic and anaerobic processes.

Table 4.1.1. Comparison of aerobic and anaerobic processes.

ANAEROBIC PROCESSES

Anaerobic process has two major stages 

(1) Acid fermentation stage.
 (2) Methane fermentation stage.

For every complex material there are many sub stages : 

(a) Hydrolysis of complex organic material
 (b) Fermentation of amino acids and sugars
 (c) Anaerobic oxidation of long chain fatty acids and alcohols
 (d) Anaerobic oxidation of intermediate products (such as short chain fatty acids except acetate)
 (e) Acetate production from CO₂ and  H₂ (homo acetogenesis)
 (f) Conversion of acetate to CH₄
 (g) Methane production by reduction of CO₂ by  H₂ (Reductive Methenogenesis)

Following group of bacteria are required for anaerobic degradition

(a) Fermentation bacteria.
 (b) H₂ Producing acetogenic bacteria.
 (c) H₂ Consuming acetogenic bacteria.
 (d) CO₂  Reducing bacteria.
 (e) Aceticlasticmethanogenesis.

Figure 4.1.2. Reaction scheme for anaerobic complex organic material .

Table 4.1.2. Major biological treatment processes used for wastewater treatment

TYPES OF BIOLOGICAL PROCESSES FOR WASTEWATER TREATMENT

For the treatment of wastewater the principle biological processes are divided into two categories: suspended growth and attached growth processes.

[A] Suspended Growth Processes 

In this process the microorganisms responsible for treatment are maintained in liquid suspension by mixing methods. The following section describes the two most widely used suspended growth processes activated sludge and aerated lagoons and one recently introduced membrane bioreactors.

Activated Sludge Process: It is the most widely used process for wastewater treatment. It consists of two sets of basins. In the first, air is pumped through perforated pipes at the bottom of the basin, air rises through the water in the form of many small bubbles. These bubbles provide oxygen from the air to the water and create highly turbulent conditions that favor intimate contact between cells, the organic material in the water and oxygen. The second basin is a settling tank where water flow is made to be very quiet so that the cellular material is removed by gravitational settling. Some of the cell material collected at the bottom is captured and fed back into the first basin to seed the process. The rest of the sludge is taken for anaerobic
 digestion.

Figure 4.1.3.  Activated Sludge Process

Aerated Lagoons and Oxidative Pond: Oxidative ponds are shallow ponds with a depth of 1 to 2 m where primary treated waste is decomposed by the microorganisms. Oxidative ponds maintain aerobic conditions, the decomposition near the surface is aerobic whereas near the bottom is anaerobic. They have a mix of conditions and are called facultative ponds. The oxygen required for decomposition is derived from either surface aeration or the photosynthesis of algae.

Membrane Bioreactors: These membranes have been designed to reduce the size of secondary treatment tanks and improve the separation efficiency. They draw water from mixed liquor into hollow fiber membranes which have a pore size of about 0.2µ m. The membranes are submerged in the activated sludge aeration tank and there is no need of a secondary clarifier or they may be present outside the aeration zone.

[B] Attached Growth Treatment

In this treatment, the microorganisms that are used for the conversion of nutrients or organic material are attached to the inert packing material. The organic material is removed from the wastewater flowing past the biofilm or the attached growth. Sand, gravel, rock and a wide variety of plastic and other synthetic material is used as the packing material. They can be used both as aerobic when partially submerged in wastewater or as anaerobic when fully submerged and no air space above it.

Trickling Filter: This is the most widely used attached growth process. It consists of a rotating distribution arm that sprays wastewater above the bed of plastic material or other coarse material.
 The spacing between the packing allows air to easily circulate so that aerobic conditions are present. The media in the bed is covered by a layer of biological slime containing bacteria, fungi etc that adsorbs and consumes the waste trickling through the bed.

Rotating Biological Contactors: It consists of a series of closely spaced circular plastic disks that are attached to a rotating hydraulic shaft. 40% of the bottom of each plate is dipped in the wastewater and the film which grows on the disk moves in and out of the wastewater