The Karachi Water and Sewerage Board (KWSB) has reportedly invited applications for short listing of firms for the reuse of treated effluent of Mauripur and Mahmoodabad wastewater treatment plants.

The two treatment plants are not working properly and the effluent quality is not appropriate.

The KWSB has not specified the effluent quality, which the effluent should meet, before it can be used for irrigation. The KWSB specifies adherence to the EPA (Environmental Protection Agency) standards. The EPA, Sindh, does not have standards or guidelines for effluent quality.

Wastewater is a valuable resource which, in view of freshwater crisis, plays an important role in water management. Wastewater reuse, directly or indirectly, is practiced globally. It is practiced extensively in the developed and developing countries.

In some countries as Jordan, Peru and Israel, all affluent from the wastewater treatment plants are used for crop irrigation. In addition to agricultural use, in some non-Muslim countries, wastewater is also used for aquaculture (cultivation of fish, aquatic plants, animal feed, fish feed, in ponds fertilized with wastewater and excreta).

In India there are over 130 wastewater-fed fish ponds, covering an area of about 12,000 hectares and; other schemes cover an area of about 73,000 hectares. Australia has the world's largest network of "sewage farms" (Weribee farms).

Even the Indus River, which is used for water abstraction, receives untreated wastewater discharges, both municipal and industrial in nature, from the cities located upstream of the river.

Potential health risk exists if crops irrigated with partly-treated wastewater are consumed uncooked, as sufficient numbers of pathogens survive in soils and on crops.

Likewise, those using wastewater for green spaces, industrial, recreational and aqua culture can be exposed to wastewater aerosols or droplets, either through inhalation or through skin cuts.

Appropriate treatment, crop restriction, methodology of application, exposure and hygiene will allow safe use of wastewater for irrigation. Certain constituents can affect crop itself, if they are present in quantities, higher than the permissible limits. As such, treated wastewater needs to meet some specified standards, before it is used for irrigation.

Sensitivity of different crops to toxic ions varies. Some crops can be more sensitive. For example, cotton and barley are more tolerant to sodium ion; wheat and sorghum are moderately tolerant to sodium, and; bean and carrot are sensitive to sodium.

Citrus crop can tolerate 600mg per litre of chloride in wastewater, while strawberry can tolerate a maximum chloride level of 180mg per litre. Cotton and asparagus are very tolerant to boron; lettuce and cabbage are moderately tolerant to boron and lemon and blackberry are too sensitive to boron (Pettygrove and Asano, 1985).

Absolute tolerance levels will, however, vary with climate, soil conditions, wastewater application methodology (surface or sprinkler irrigation), and cultural practices.

Excessive salinity will damage the crops. Sodium poses soil permeability problems. Excessive amount of free available chlorine (greater than 0.05 mg per litre) will cause leaf-tip burn and damage some sensitive crops.

In treated wastewater, however, most chlorine is in a combined form, which may not cause crop damage. Symptoms of excess boron include leaf-tip and marginal burn, leaf cupping, yellowing of leaves (chlorosis), red and blue leaves (anthocyanin), rosette spotting, premature leaf drop, branch dieback and reduced growth (Petty grove and Asano, 1985).

Typically, municipal wastewater contains adequate amount of nutrients. However, over abundances of nitrogen can cause crop lodging and excessive vegetative growth (eutrophication, in case of surface water bodies, e.g., lakes and reservoirs).

Excessive nitrogen results in excessive foliage growth. Excess nitrates in forage crops may result in the production of nitrous oxide gases. These gases can be deadly to humans and farm animals in enclosed areas (Environment Canada, 1984).

While, potassium and phosphorus are, generally, non-toxic to plants, heavy metals pose significant problems to crops. Absorption of excessive amount of heavy metals by plants restricts their growth and can cause injury. Severe phytotoxicity may kill plants. Zinc, copper and nickel cause phytotoxicity.

Excess copper and nickel cause injury to plant roots. Copper and nickel toxicity symptoms are generally expressed as yellow (chlorotic) young leaves due to induced iron deficiency. Zinc is trans-located more freely to the leaves than copper and nickel.

Although, zinc can injure roots and induce chlorosis, its toxicity is usually manifested through injury to older leaves and, reduction of plant growth through interference with biological processes.

Heavy metal of most concern, when it comes to the application of wastewater for irrigation is cadmium. Cadmium is highly toxic and is phytotoxic to plants. The uptake of cadmium differs greatly from crop to crop.

Uptake of cadmium by rice would be around 5mg per kg, dry weight, whereas, in case of spinach and turnip, it could be around 160mg per kg, dry weight (Environment Canada, 1984).

Excreta and wastewater generally contains high concentrations of pathogens. These are passed on to others either through mouth or skin. The infections are largely controlled by taking appropriate measures and by personal hygiene. The World Health Organization (WHO-1989) had divided infections into five categories, according to their environmental transmission characteristics.

Various types of treatment plants exist which can treat the wastewater. Some of these are: activated sludge, trickling filters, aerated lagoons, oxidation ponds, anaerobic contact process, anaerobic filters, rotating biological contactors and oxidation ditches.

The choice of treatment system is influenced by cost, availability and land cost, low-land area requirements, odour problems, availability of indigenous technology, use of minimum electrical and mechanical components, ability to produce the desired effluent quality, simple operation and maintenance and ability to withstand toxic metals concentration.

Based on experience, aerated lagoons seem to be the most appropriate treatment system. According to Shuval et al. (1986), aerated lagoons have been reported to provide removal rates of 60 to 99.99 per cent for total coliforms and 99 per cent for faecal coliforms, total bacteria, Salmonella typhi and Pseudomonas aeruginosa.

Conventional wastewater treatment plants are efficient in removing the parameters, and pollutants, which are of interest in environmental pollution control (biochemical oxygen demand (BOD), and suspended solids).

They are not every effective in the removal of pathogens, such as helminths, unless suitable unit operations are added; or, if the quality of wastewater is further improved after treatment by prolonged storage, conveyed over-long distances in river or canal, or by dilution in rivers.

In order to ensure that the impacts of the use of wastewater for irrigation are minimal on crops and human health, the quality of effluent must be evaluated to ensure its proper use.

Use of wastewater, whether it is raw or partly-treated, will pose significant health risks to humans. Uptake of toxic components of wastewater would be injurious to crops.

These crops, when eaten undercooked, would again pose health risks to human. It is essential that the wastewater is properly treated before its use for irrigation. Maintaining personal hygiene is an important adjunct of application of wastewater for irrigation, if the risks to the diseases are minimized.

While the initiative taken by the KWSB is highly appreciated, as it will take off some pressure on drinking-water, it should proceed on this issue very carefully, so as to preclude any outbreak of disease as the result of wastewater application.

The KWSB needs to prescribe standards for effluent in the first instance, which the selected firms should meet, before treated effluent can be used for irrigation. Technical guidance can be provided to the KWSB in developing the standards for effluent quality and for wastewater sludge.