PLANNING DOMESTIC WASTEWATER TREATMENT

PLANNING DOMESTIC WASTEWATER TREATMENT

In order to guarantee availability of water, it is essential for the local government to plan and build water supply schemes that supply water to the community in sufficient quantity that is potable in terms of quality. Water which is safe and potent for drinking is called wholesome water. A lot of money is needed for planning, designing and executing a wastewater treatment system. A sewerage system should be properly and skillfully planned and designed in order to efficiently remove sewage to the point of disposal and avoid health hazards to the population. A sewerage system requires enormous capital investment as well as recurring repair, maintenance and operational (RMO) expenses. In order to plan a wastewater treatment plant, the topography of the region plays a vital role. The plant should be located at the lowest point thereby reducing the installation of pumps and other energy consuming equipment. It should be located away from the community to reduce odour problems and minimise spread of vector borne diseases. The wastewater treatment facility should be located close to land with porous soil for disposal or should be close to a water body for economic disposal of treated waste in accordance with local rules and regulations.

• Outline of unit processes

Wastewater can be treated by physical, chemical or biological methods. Based on the method used to treat the waste, the treatment methods are classified as unit processes and unit operations. The types of treatment used to remove contaminants by addition of chemicals or use of biological mass or microbial activities are known as unit processes. If the treatment involves use of physical forces, it is known as unit process.  A few important treatment methods that fall in this category are:

1. Screening-- removes floating materials
2. Mixing-- mixing the contents of wastewater for various purposes
3. Flocculation-- coalescing of particles to form bigger aggregate flocs
4. Sedimentation-- removal of solids or biomass by gravity settling
5. Flotation-- removal of solids by floating them on the liquid
6. Elutriation-- washing of sludge to remove alkalinity
7. Filtration-- removal of suspended solids by filtering them from liquid
8. Heat transfer and drying-- removal of water or moisture from sludge

• Different types of treatment methods

Based on the type of agent used, unit processes are further classified as:

1. Chemical unit process: These processes involve removal of contaminants by adding chemicals. Processes of this type are:
1. Chemical neutralization - involves control or adjustment of system pH
2. Chemical coagulation - involves removal of colloidal particles by chemical destabilization and flocculation
3. Chemical precipitation - involves enhancement in removal of suspended solids, phosphorus, heavy metals and BOD in specific system conditions
4. Chemical oxidation - involves removal of grease, ammonia, BOD, COD and odour
5. Chemical disinfection - involves killing pathogens in influent and treated effluents

Note: Chemical processes are not normally provided in conventional treatment of domestic wastewater due to high cost of chemicals. 

• Major treatment methods that fall under biological unit process are:

1. Suspended growth process and
2. Attached growth process

Special cases involving removal of excessive nutrients and liquid or gaseous impurities the following advanced treatment methods are used:

1. Biological nitrification-denitrification
2. Reverse osmosis
3. Ion-exchange and
4. Ultrafiltration.

• Primary treatment
• The primary treatment system is preceded with a preliminary treatment system for the removal of floating substances and large inorganic matter causing operational problems in primary and secondary units in the treatment system. The preliminary treatment system consists of:

1. Sump and pump unit
2. Approach channel
3. Screen chamber
4. Grit chamber and
5. Skimming tank (oil and grease traps)

The sump and pump unit and approach channels are NOT treatment units. They can be called - 'holding and conveying' units as they are primarily involved in collection, lift an conveyance of wastewater.

• Screening - Screening is the first operation in a sewage treatment plant. It consists of screens of several sizes to trap and remove floating matter. The floating material if not removed, have the potential to choke pipes and adversely affect sewage pumps. Thus the main purpose of providing screens is to protect the equipment from damage due to floating material. Screens are preferably placed before the grit chambers or may be accommodated in the body of the grit chamber. Screens may be coarse, medium or fine. Material collected on screens is removed using rakes. Screens may be classified as fixed screens or movable screens.

• Neutralization - Neutralization consists of neutralizing the excessive acidity or alkalinity of wastewater by adding alkali or acid respectively. This takes place in either the equalisation tank or a separate neutralisation tank.

• Equalization - Equalisation consists of holding wastewater for a pre-determined time in a continuously mixed basin in order to produce wastewater of uniform characteristics. This is necessary if the characteristics of wastewater varies throughout the day.

• Flocculation - When certain chemicals (Ex - coagulants like alum) are added to water, the tiny suspended colloidal particles are trapped in a gel like precipitate formed by the reaction between water and the coagulant. This is called a 'floc'. Tiny flocs come and bind together (coalesce) forming large flocs that settle at the bottom. This process is called flocculation.

• Sedimentation - This is one of the oldest method of removing suspended particles in water. It is based on the principle that suspended particles settle to the bottom when water is 'stationary'. However, water cannot be made to stand still in a treatment facility and hence, in order to achieve the desired effect, the velocity of water is reduced and the distance of travel from the inlet to the outlet is increased. Thus the particles settle to the bottom of the sedimentation tank from where they can be removed.

• Floation - Floatation consists of creation of fine air bubbles in the tank by introduction of air into the tank from the bottom. The rising air bubbles get attached to the fine suspended particles, increasing their buoyancy and lift them to the surface of the liquid and thus they canbe removed by skimmimg.

• Nitrification and Denitrification Systems - Nitrification is the biological oxidation of ammonia to nitrite and subsequently to nitrate. This involves a type of bacteria called nitrosomonas.

• Environmental impact and other considerations in planning treatment facilities
• Wastewater treatment plants require large space
• After significant treatment, the effluent from  the treatment plant is discharged on land or into near-by water body causing pollution.
• The life-cycle environmental load of the treatment facility should analysed
• The construction and operational phases of the treatment facility should be given due importance
• Damage to ecosystem quality should be assessed including ecotoxic emissions along with the combined effect of eutrophication and acidification