Water Purification : By Bioprocessing and Chlorine Disinfection

1. By Bioprocessing:

For maintaining purity of municipal/corporation water supplies its chlorination is highly effective in purification of water from microbial contamination thereby preventing spread of infectious diseases.

Not only municipal/corporation water supplies but also other water supplies must pass through a series of purification steps before pipe line supply to make it safe for either human consumption or for process industry and agriculture requirements.

In some cases, however, the water resource may be sufficiently pure to require only treatment with a disinfectant like chlorine, chlorocresol etc. depending on the quality of treatment desired and the purpose.

In many instances the condition of water resource is such as to require normally three stages of primary purifications namely, sedimentation, filtration followed by disinfection.

However, in recycling of used water in industries or for their disposal in natural water ways stringent treatment methods are needed to make waste water/liquid effluent safe for recycling, for aquatic life and prevention of water basin filling up by metallic or other solids. Reference is therefore made to water resources and the danger of their pollution.

(i) Oxic Bacterial Process Biotechnology:

It is well reorganized that there are limits to the self purifying ability of a natural body of water. This limit pertains to both capacity and the speed of self purification. In recent industrialized societies necessity of development of rapid man-made water purification has become obvious.

In a more recent process a mix of various strains of bacteria has been used to assimilate organic matter natural as well as man-made consuming oxygen and producing chiefly CO₂ and H₂O.

The technological success greatly depends on the type of water and a pollutants, their amount and concentration, the consistency of these determinants and last but not the least, on the speed of bacterial assimilation and the presence of inhibitory ingredients.

Many process biotechnology have been developed in laboratories and pilot plants. Their operational reliability is of utmost importance, both in order to ensure continuous supply and necessary quality of treated clean after the purification process. Oxic biological nitrification of organic present in polluted water is also one of the major concerns in this bioprocessing to avoid eutrophication.

(ii) Anoxic Bacterial Process Biotechnology:

Many microbes not only survive but even grow rapidly when starved of oxygen. For survival these microbes avail gene machines which encode enzymes essential for anoxic growth. The most successful bacterial groups can use nitrate as a substitute for oxygen/air.

In anoxic stage of water purification/treatment less energy is derived from nitrate reduction than from conventional cell respiration, so genes, for nitrate reduction are activated only when nitrate is present but oxygen is unavailable. However, the product of nitrate reduction is very toxic. Actually, nitrate respiring bacteria in water purification does make provision for removal of nitrate, so formed.

(iii) Integrated Oxic-Anoxic Process Biotechnology: Consideration:

In the water purification bacterial nitrogen metabolism is an important key to clean water. Bacterial metabolism of three molecular species: ammonia, nitrate and nitrogen, dominates the oxic-anoxic integrated biological nitrogen cycle. In oxic biotechnology nitrification plays major role while denitrification fixation governs anoxibiosis in the process biotechnology. A striking balance between oxic-anoxic bioprocessing and water pollution is important so that the objective of waste water purification is not defeated.

(iv) Oxic-Anoxic Design Scheme:

In order to overcome the problem of eutrophication in waste water purification nitrogen content in water must be removed otherwise, it will cause BOD increase. The removal of nitrogen from waste/water can be done by appropriate schemes of oxic-anoxic design systems for nitrification/ denitrification. Operational characteristics of three schemes for designs of nitrification and denitrification have been described below (Fig. 11.15).

Scheme 1:

This scheme depends upon the endogenous respiration of the activated sludge to achieve denitrification.

Scheme 2:

Here a portion of the influent waste water is by-passed to the denitrification tank to provide food for the facultative organisms thereby increasing the respiration rate and hence the denitrification rate. Scheme 3: It uses influent waste water which is nitrogen deficient as a food source for denitrifying organisms. The waste water should contain a readily available carbon source.

(v) Relative Advantages and Disadvantages of the Schemes:

In scheme 1 while the bioprocessing achieves a low nitrogen effluent, the slow rate of denitrification under endogenous respiration conditions results in a large denitrification tank. In scheme 2 while some reduces the required size of the denitrification tank it has the disadvantage of increasing the unoxidized nitrogen in the treated effluent and in most cases increasing the effluent BOD.

While scheme 2 practice will not contribute nitrogen to the effluent, careful controlled operation is required to avoid increasing the effluent BOD. Experimental results and experience indicated that economic use of this process scheme necessitates increasing the respiration rate and hence denitrification rate in denitrifying unit. It is also required for only carbonaceous BOD removal. If higher O₂ level is maintained in the aeration tank for maximum nitrification rate, the power requirement will be about two and half times that required for conventional activated sludge process operation.

(vi) Acceleration of Denitrification:

Biodenitrification for removal of nitrate and nitrite could be accelerated by a using appropriate amount of methanol. Based on stoichiometric equations of biodenitrification it could be computed that 1 mole NO₃ is equivalent to 5/6 mole methanol.

When the waste water which is to be treated contains dissolved oxygen (DO) it needs to be removed before denitrification step. This has been accomplished by adding extra amount of methanol. From these considerations the total amount of methanol (C_m) requirement could be computed by the following relation:

C_m = 2.47 N₀ + 1.53 N_i + 0.87 DO

Here N₀ is the initial nitrate, N_; is the initial nitrite and DO is the initial dissolved oxygen concentration.

2. Chlorine disinfection and biohazard:

Purification of drinking water by chlorine disinfection is an age old process. It is very effective in killing hazardous microorganisms in potable water. However, bio-molecular design engineering concerns of water chlorination hazard are being known in more recent years.

Over-chlorination in water is hazardous to human health. The relationship between the concentration of available chlorine and the time taken to kill the organisms in water is exponential i.e.

cⁿt = constant (k) (11.20)

It has been stated that in human lymphocytes, chlorinated humic substances produced DNA strand cleavage but only at a concentration that caused cytotoxicity.