Wastewater Treatment Practices in Pakistan (Part 3)

1.1-Secondary Treatment

In this method we remove dissolved and suspended particles. This method is specially designed to remove biological content, derived from human waste, organic and industrial waste. Massive types of wastewater treatment plants use biological techniques for the settled sludge liquid. For this purpose we should have proper information about the oxygen and substrate content of the main source. For example, bacteria and several species of Protozoa used for the biodegradable process of soluble organic waste like fats, nucleic acids, vitamins, etc. We should require a separation process for the removal of biological microorganisms before it passes to the tertiary level. The secondary treatment are further has two parts on the basic of biomass that are present in the wastewater.

• Fixed Film Method

• Suspended Growth Method

1.1.1-Up Flow Anaerobic Treatment Reactor

This method is successfully used to treat different kinds of domestic and industrial solid waste generation. We simply conclude the procedure by describing its phenomena in which the water passes from sludge making channels. Sludge contains small granule like particles. Then we add some amount of influents from the bottom of channels where they react with the substrate particles. The velocity of the flowing influents must be slow that helps substrate to react properly with the biomass. It is highly recommended treatment system where no excess sludge production was made. The resting biomass is finely converted into biogas at the end of the reaction.

1.1.2-Bioreactors

A)-Membrane Bioreactors:

In this process we combined the activated sludge treatment with a membrane liquid solid separation process. The membrane contains low pressure micro filtering agents or we called them ultra-filtration membranes that further used for water tertiary treatment. We immersed these membranes in the aeration tank. It is highly effective method because it overcomes the limitations of sludge production and separate the sludge in a conventional activates system. This system stops the bioreactors to activate with higher mixed liquid suspended particles and the biomass concentration allows easily removal and separation of soluble biodegradable particles. Maximum time used for sludge retention is usually 15 days which ensures the nitrification reactions to occurs without any limitation.

B)-Rotating Biological Contractors:

These are the mechanical procedures of secondary treatment which allows the biodegradation of organic material. These contractors were first installed in America where they have been developed into effective operating units. It contains a rotating disk for the complete growth of bacteria and other microorganisms. Then these microorganisms are used for the biodegradation of organic matter. To effectively procedure the reaction microorganisms need complete supply of oxygen from the environment when needed. Biomass from contractors is further passes to fine clarifiers channels, where microorganisms settle down at the bottom of the contractor.

Example:

A similar kind of filtration method is also installed at homes for aquarium filtration and purification method. The water flows upward from the aquarium bottom then passes towards the fiber mesh wheel for filtration process, and then returns back to the bottom. This fiber mesh contains a thin film of microorganisms that degrade the suspended organic particles from the water.

C)-Lagoonings:

It provides chance for further biological treatment through storage process in large manual tanks. These tanks are aerobic and contain species of macrophytes especially reeds for biodegradation of organic material.

D)-Constructed Wetlands:

These constructed wetlands contain engineered reed beds for further improvement of the sewage water. These wetlands have high aerobic tanks and sometimes used in contrast to secondary treatment. Their membranes have fine porous surface where the suspended substrates are collected. During flow treatment, the water remains in contact with aerobic and anaerobic zones. Rhizomes are used to create an aerobic environment. This is an effective method used for the treatment of domestic and municipal waste water from past few decades.

E)-Nutrient Removal:

Sewage waste water contains a high amount of nutrients nitrogen and other particles. If we disposed the wastewater which is enriched in nutrients into natural water bodies, these cause eutrophication. It is a process in which microorganisms feed on the nutrients and grows continuously on the surface of water and blocks the sunlight to reach at the bottom of the lakes or rivers. Simply it cause algal blooms, they are high in number and some of them died. These dead algal blooms degrade by some colonies of bacteria that use oxygen, cause ultimately shortage of oxygen in water. Oxygen depletion is one of the main causes of death of marine plants and animals. To overcome this problem, there is high need to remove these nutrients before it enters into any water body. For this purpose we use varieties of wastewater treatment processes.

1-Nitrogen Removal:

Through biological oxidation of nitrogen from oxides of nitrogen into nitrates is highly effective and most feasible method. Ammonium converts into nitrates by the action of specialized microorganisms called Nitrobacter or by Nitrospira. Comparative to microorganisms we also use contractors, weed-beds or membrane filters for the effective removal of nitrogen from waste water. As the denitrification is a reduction reaction, so we need a donor atom along with other species that initiates the reaction. It totally depends upon the biomass present in the waste water. Sometimes this method is referred as tertiary treatment for waste water improvement.

2-Phosphorus Removal:

Just like nitrogen removal, phosphorus removal is also a crucial step. It also favors the algae growth in fresh water bodies. Phosphorus can be removed by a special “Enhanced biological phosphorus removal” method. It is a biological method that uses bacteria especially designed for the accumulation process of phosphorus. These microorganisms effectively react with the phosphoric compounds and start accumulating of phosphorus at the bottom of the reactor. Then these bacterial species can be separated from the rest biomass, which have a high fertilizer capacity.