Wastewater Treatment

Wastewater Treatment Methods Wastewater treatment system consists of a combination of different treatment units, which either reduce the quantity of impurities or qualify wastewater to undergo action in next treatment unit. Each treatment unit employ one or combination of physical, chemical or biological measures. Based upon the type of measure used, treatment method can be broadly classified into Unit Operations and Unit Processes.

Unit Operations Treatment measures in which the application of physical forces predominates are known as unit operations. Major treatment measures falling under this category are as follows: i. Screening ii. Mixing iii. Flocculation iv. Sedimentation v. Floatation vi. Elutriation vii. Vacuum filtration viii. Heat Transfer and drying

Screening Screening is the first operation at any wastewater treatment works. This process essentially involves the removal of large non-biodegradable and floating solids that frequently enter a wastewater works, such as rags, papers, plastics, tins, containers and wood. Efficient removal of these constituents will protect the downstream plant and equipment from any possible damage, unnecessary wear & tear, pipe blockages and the accumulation of unwanted material that will interfere with the required wastewater treatment processes. Wastewater screening is generally classified into either coarse screening or fine screening. Screens may be manually or mechanically cleaned, with only the older and smaller treatment facilities using manually cleaned screens as their primary or only screening device. Coarse screens are typically used as primary protection devices, and usually have openings of 10mm or larger.

Fine screens are used to remove material that may cause operation & maintenance problems in downstream processes, particularly in systems that lack primary treatment. Typical opening sizes for fine screens are 3 to 10mm.

Mixing Mixing is required in various units of a treatment plant, and it can be achieved by rotating mixers, static mixers or creation of a hydraulic jump.

Flocculation Flocculation is "a process of contact and adhesion whereby the particles of a dispersion form larger-size clusters."

Sedimentation Sedimentation is the process in which suspended particles settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them.

Floatation The process of floating a solid or liquid impurities in wastewater due to their low densities is called floatation.

Elutriation Elutriation is a method for separating insoluble solids into different particle sets based on their size, shape and density, using different methods or forces. Normally the forces used for elutriation includes, gravity, buoyancy, electrochemical forces or centrifugal forces.

Vacuum filtration Filtration is a technique used to separate a solid from a liquid. The solid is separated from the liquid phase by passing the mixture over a filtering media. Vacuum filtration is process in which speed of filtration process is increased by sucking liquid through membrane. This process is normally used for sludge dewatering.

Heat Transfer and drying Heat transfer and drying are simultaneous processes normally used in sludge processing and dewatering. In first step, heat is transferred to refuse due to which water in refuse start evaporating which is called drying.

Unit Processes Type of treatment in which removal of contaminants is brought about by addition of chemicals or use of biological mass are called unit processes. Therefore, unit processes are further categorized into chemical and biological unit processes.

Chemical Unit Processes Reduction or removal is brought about by means of chemical reactions by adding chemicals and providing suitable environment which facilitates the desired reactions. Major treatment methods falling under this category re as follows Chemical neutralization To control or adjust system pH. Chemical coagulation To remove colloidal particles by chemical destabilization and flocculation Chemical precipitation To enhance the removal of suspended solids, phosphorous, heavy metals and BOD in specific system conditions

Chemical oxidation To remove grease, ammonia, BOD, COD and for odor control in particular requirement. Chemical disinfection To kill pathogens in influent and treated effluents

Biological Unit Processes The wastewater treatment methods in which treatment purpose is achieved by microorganisms. Major treatment methods falling under this category are classified as follows. Suspended growth process Microorganisms, performing biological activity are in suspended and evenly mixed in wastewater. Some examples of this category are Activated Sludge Process Aerated Lagoons Oxidation Ponds Aerobic and Anaerobic digesters

Attached growth process Microorganisms, performing biological activity are attached and make colonies to some medium. Wastewater pass through the medium and microorganisms attached to medium extract solids/contaminant and fulfil treatment purpose. Some examples of this category are Trickling Filter Rotating Biological Contractors (Bio Rotors) Bio Towers Up-flow filters

Wastewater Treatment Systems Set of combination used from available unit operations and processes for wastewater treatment can be categorized in four treatment steps i. Preliminary Treatment ii. Primary Treatment iii. Secondary Treatment iv. Tertiary Treatment / Advanced Treatment

Preliminary Treatment Objective: Preliminary treatment fulfil following purposes Receive influent Flow Measurement Screening to protect next treatment units from large size solids Pumping, as required to lift wastewater above ground level so that wastewater flow should be derived from gravity in following treatment steps Grit Removal to protect mechanical equipment from abrasive wear. A schematic process flow diagram of preliminary treatment is as follows

Primary Treatment Objective: To reduce organic load on secondary treatment processes. Primary treatment removes heavier solids from wastewater by sedimentation. Primary clarifier reduces 60 – 70% of suspended inorganic solids and 30 – 32% of organic solids A schematic process flow diagram of preliminary treatment is as follows

Secondary Treatment Objective: To convert organic material (BOD) into solid biomass for removal in secondary clarifier (SST). Several Treatment alternatives exist including

Activated Sludge (Suspended Growth)

Trickling Filter (Attached Growth)

Tertiary Treatment / Advanced Treatment Objective: To further reduce the amount of residual impurities in effluent of secondary treatment Advanced treatment is normally provided in following cases. i. The quality of conventionally treated wastewater (secondary effluent) is unsuitable for final disposal requirements into receiving water body. ii. The concentrations of residual organic materials or suspended solids require further reduction or complete removal for specific reuse or recycling of wastewater after secondary treatment iii. The concentrations of nutrients like nitrogen & phosphorous is high for final disposal or reuse / recycling of wastewater.

Functions of Treatment Plant Units Major functions of treatment units shown in schematic diagram of wastewater treatment plant are as under i. Sump and Pump Unit To collect community wastewater in sump unit and pump it to next units at raised level so that flow is maintained by gravity. ii. Screen To remove large floating material in wastewater to protect next treatment units from damaging and clogging iii. Grit Chamber To remove high density suspended particles by settlement iv. Skimming Tank To collect floating solids, oil and grease from water surface

v. PST (Primary Sedimentation Tank) To remove fine suspended inorganic and settle able organic solids (BOD) and floating scrum. vi. Aeration Chamber Acts as biological treatment unit in which microorganisms in is suspended form contribute toward treatment. Much of COD and BOD is removed in this process. vii. SST (Secondary Sedimentation /Settling Tank) To thicken and segregate biological sludge from wastewater viii. Sludge Digester To treat sludge before final disposal

Selection of Treatment Plant System Selection of a particular treatment system and choosing a suitable combination of treatment units depends upon following factors 1. Degree of treatment to bring quality of final effluent. This means that type and amount of impurities in influent that need to be reduced to a certain level. 2. Regulatory /legal requirement of amount of impurities in effluent. 3. To ensure that final effluent is safe for end disposal or acceptable for specific reuse. 4. Topography and space available for treatment plant site. 5. Availability or non-availability mechanical and skilled personal to handle treatment plant.

It is also important to consider following points while selecting a specific treatment system Reduction of inorganic material component of wastewater is much easier and cheaper than the removal of organic contents of wastewater Removal of suspended solids require from wastewater requires lesser time and effort than colloidal and dissolved solids.