Use of treated waste water can bring food security

One of the biggest problems the agriculture is facing today is scarcity of water and with time it will be still scarcer. Increased water withdrawals to meet the requirement of urban population, industries and expansion of sewerage facility produce greater quantities of wastewater. With the current emphasis on environment, health and water pollution issues, wastewaters generated need safe disposal. Due to scarcity of good quality water and competition from other remunerating sectors, availability of good quality water to agriculture is reducing at an alarming rate. Under such conditions, water from any source including wastewater is to be used effectively and judiciously in agriculture not only to avoid the pollution of valuable resources but also to exploit their irrigation and nutrient potential. India with its 4.2% share of global water resources is supporting 16.7 per cent of global population. Nearly 85% of India’s fresh water resources are being utilized in agriculture and the balance 15% in domestic and industrial sectors.

Despite of abundant global availability, only a miniscule fraction of the water available on the earth surface is used by the mankind; other being too saline to use or is locked at places and in forms that man is unable to make its use under the current technologies available. Although, agriculture is the major user of water claiming more than 70 per cent of water withdrawn from rivers, yet it is unable to compete economically for scarce water resource as industry and domestic sectors can pay more for water and can earn more per unit of water used. But food security being closely linked with water availability to agriculture, it will continue to be the major beneficiary of water resource development. At the same time, the sector would be increasingly called upon to use waters of low quality. Although both the quantity and the quality aspects of water are important but in this paper we review national scene with respect to water supply as related to its quality and its use in crop production.

In agriculture or for that matter in any sector, there is no such thing as "pure" water because even the rainwater will have many impurities as it falls through the air to the earth or flows on the surface of the earth or moves underground to meet the groundwater. The rainwater usually takes on the characteristics of the materials it has encountered on its way till it meets its final place of storage. Generally speaking, surface water doesn’t have enough impurities that would affect the utilization of these water for most practical purposes in domestic, industrial or agriculture. But on the other hand, groundwater due to excessive dissolution of salts could turn saline/sodic and may become unfit for use in domestic, industrial or even agriculture unless managed with care. Such groundwater is generally referred as naturally occurring wastewaters and is a characteristic of the arid and semi-arid regions where groundwater is usually saline/sodic in nature. Saline and sodic water contains salts that impair plant growth but rarely contains pathogens. However, short and long-term use of such water can lead to soil salinization/sodication. Quality of water is usually defined with respect to specific use. These are only preliminary guidelines but more specific and detailed guidelines are also available for such uses or some other uses such as land disposal and disposal in the water bodies. Central Soil Salinity Research Institute has developed detailed guidelines for use of saline/sodic including toxic groundwater for irrigation. Any water to be useful to the society has to have the parameters characterizing it, in a desirable proportion depending upon its use.

Sources of wastewater

In general wastewater is grey domestic (without human excreta), black domestic with human excreta, or industrial wastewater. Domestic wastewater consists of discharges from households, institutions and commercial buildings while industrial wastewater is the effluents discharged by variable manufacturing units and food processing plants.

Besides the naturally occurring saline/sodic ground waters, other kinds of wastewaters from domestic and industrial sectors have received prominence mainly because of shrinking water resources across the globe as well as their nuisance value in polluting the surface and groundwater resources. In the real term, the term "wastewater" is a broad, descriptive term that designates liquids and waterborne solids from domestic, industrial or commercial uses that have been adversely affected in quality by anthropogenic influence and have limited applications for use unless appropriately treated/managed.

Although not separable, two categories of wastewaters i.e. domestic wastewaters and industrial wastewaters have emerged mainly because of major differences in their quality. Domestic wastewaters originate principally from domestic, household activities but will usually include waters discharged from commercial and business buildings and institutions. Nearly 80 % water supplied to domestic purposes drains back into municipal sewer system. Surface and storm waters may also be present. Domestic wastewaters are usually of a predictable quality and quantity. These domestic wastewaters are sometimes sub-grouped as dark and grey depending upon the source in the household. Human wastes consisting mostly of feces and urine, which becomes part of the wastewater through toilet flushing is called dark water. On the other hand, household wastes derived from home laundry operations, bathing, kitchen wastes, from washing and cooking foods and dishwashing etc. is termed as grey water. The grey water will contain synthetic detergents but the bacterial contamination would be to a much lesser degree. Whereas grey water would pose minimal problems in its reuse particularly in agriculture such as irrigating lawns and kitchen gardens, the disposal assumes significance because dark water is usually discharged with grey water.

Wastewater Quantity

Total wastewater generated from all major industrial sourced in India is 83,048 MLD that includes 66,700 MLD of cooling water generated from thermal power plants and out of remaining 16,348 MLD another 7,275 MLD is generated as boiler blow down water and overflow from ash ponds. Second bigger contributor is small scale engineering industries. Of this, electroplating units are most polluting units. Other significant contributors to industrial effluents are paper, textile, steel and sugar industries. Latest estimates suggest that about 22900 MLD of domestic wastewater is generated in different cities of our countries. Information on wastewater characteristics and its flow rates are important in designing and operation of collection and disposal facilities as well as in knowing their irrigation potential for agricultural purposes. Use of wastewater for irrigation will avoid pollution of water bodies and environmental hazards like public health and foul smell.

Waste waters generated in huge quantities from municipalities and agro-based industries in Haryana were a****sed for their quality, irrigation potential, nutrient supply and possible health hazards for formulating efficient, eco friendly technologies and measures required for their sustained use in agriculture. The farmers in peri-urban areas largely depended upon these waters for their livelihood and meeting most of the crop nitrogen and phosphorus requirements. Estimates are that sewage waters can annually irrigate about 1.5 Mha (million hectares) of land area and has a potential to contribute about one million tonnes of nutrients and 130 million man-days of employment. Irrigation with various sewage or sewage mixed with industrial effluents resulted in saving of 25 to 50 per cent of N and P fertilizer and higher crop productivity over the normal waters. However, long-term sustainability of irrigation with such waters depends on several factors such as site-specific soil, climate, crop, application techniques and socio-political environment. Indiscriminate and unscientific application of wastewater led to higher pathogen load, deterioration in crop quality and contamination of soil and groundwater..

Composition of wastewater

Though actual composition of wastewater may differ from community to community but all municipal wastewater contain; organic matter, nutrients (N, P, K and micronutrients), dissolved inorganic minerals, toxic chemicals and pathogens. However, the final composition of raw wastewater depends on the source of water supply, types and numbers of industrial units discharging effluents, and level of treatment given. Routine measurements of municipal wastewater pertain to water pollution parameters like BOD, suspended solids and COD. But agriculturally important chemical characteristics as elemental composition and compounds that affect soil properties and crop growth are sporadically monitored.

The major contributors of pollution in terms of BOD are distilleries followed by paper mills. Distillery effluents are very concentrated and thus difficult to treat. Paper and board mills also generate heavy organic pollution load. Other significant contributors of organic load are sugar mills and tanneries. Industries generating chemical pollution can be divided in two categories i.e. those generating high TDS as wastes of pharmaceuticals, rayon plants, chemicals, caustic soda, soap, detergents and smelters etc. while second type include those units which generate toxic wastes like pesticides, smelters, inorganic chemicals, organic chemicals, steel plants and tanneries. Distilleries, textile units, pharmaceuticals and rayon plants contribute to TDS; whereas thermal power plants followed by paper mills and tanneries generate suspended solids loads.

Fertilizer plants generate toxic wastes as cyanide and arsenic. Steel plants and oil refineries contribute to phenols while engineering units, refineries and vanaspati (hydrogenated vegetable oil) industry release oils and greases in the environment. Tanneries add Cr and fertilizer units also add fluoride to the system. Similarly, caustic soda units release Hg in environment.

Wastewater disposal and use for irrigation

Developed countries with surplus funds regard wastewater treatment as vital to protect human health, environment and prevent the pollution of surface water bodies such as lakes and rivers. But for most developing countries, this solution presents insurmountable problems and is prohibitively expensive. Under these conditions, applying wastewater to agricultural lands is a more lucrative/economical alternative as it provides a reliable source of water for agriculture. Water does not lose its utility after one/two use(s) and has the potential of recycling for few activities that don’t require good quality water. Thus, use of wastewater in agriculture is a low cost method to dispose of municipal wastewaters compared to its land disposal or disposal in lakes/rivers/sea. In the Indian context many of its rivers have already turned into drains during season other than monsoon. The positive and negative effects of wastewater irrigation are summarized below. If the negative issues could be appropriately addressed then many of the reservations for wastewater use could be countered.

Millions of farmers around the globe irrigate with wastewaters either because they have no other source or it is cheap source of their livelihood. No precise estimates of wastewater irrigated area are available for India yet the widespread use could be gauged from the fact that along one river, the Musi in Andhra Pradesh, alone an approximately 40,500 ha area is irrigated with wastewater. According to another report, the rural areas downstream of Vadodara in Gujarat, India, present an interesting case where wastewater supports annual agricultural production worth Rs. 266 million (Bhamoria, 2005). Critics however, aver that irrigation with untreated wastewater can represent a major threat to public health (of both humans and livestock), food safety, and environmental quality. The microbial quality of wastewater usually measured by the concentration of the two primary sources of water-borne infection – faecal coliforms and nematode eggs might not suffice in the long run as a range of viruses and protozoa pose additional health risks. In sum, wastewater is a resource of growing global importance and its use in agriculture must be carefully managed in order to preserve the substantial benefits while minimizing the serious risks.

Positive and negative effects of wastewater irrigation

Pollution vis-à-vis public health

Studies at Haryana revealed that TDS in sewage ranged between 0.6 – 3.3 g/L and these are on higher side than permissible levels, this could be because of collection wastewater through open drains containing considerable quantities of clays and silt getting mixed with sewage. BOD and COD of wastewater ranged from 176 to 345 ppm and 233 to 457 ppm, respectively. Ambala sewage had maximum BOD and COD. Most of nitrogen in wastewater is found in NH₄-N (39 ppm). These parameters indicate that these should not be allowed to be disposed in water bodies as these may deteriorate aquatic environment. Considering the agroclimatic conditions of Haryana, the most of wastewater generated are suitable for irrigation purposes.

Health hazards associated with disposal of raw or partially treated sewage are monitored in terms of E. coli, F. coli, total bacterial counts, Salmonella and Shigella. The populations of E. coli and F. coli are considerably higher 4 x 106 per 100 ml of wastewater. Bacterial counts and fungi were also high than permissible levels. Although other pathogenic bacteria were not detected but presence of high levels of coliform bacteria indicates towards health hazards from use of these wastewater.

Nutrient potential

As a matter of fact, additions of macro- and micro-nutrients and organic matter for conditioning of soils are inseparable from sewage irrigation and thus their use can diminish their requirements for fertilizers, sewage from different districts in general are rated high in terms of plant nutrients contents. Contents of major plant nutrients, i.e., N, P and K, averaged 45.9, 6.9 and 62.4 ppm, respectively. Samples analyzed for micronutrients status indicate 0.17, 1.01 and 0.024 ppm, respectively of Zn, Fe and Cu. Thus potential for the supply of major nutrients like N, P, K with irrigation is 34.4, 5.2 and 46.7 kg/ha in addition to 130, 760 and 20 g of Zn, Fe and Cu, respectively.

Supplied nutrients are expected to be utilized more efficiently as these are added in splits and the total nutrients added during crop growth period will be sufficient for successful crop production. Also it is a fact these effluents contain appreciable amounts of organic matter that improves soil conditions. From the concentrations of respective elements and quantum of sewage, it is estimated that these effluents have potential to contribute about 56.4 Mg/day and 20,872 Mg/year c**ulatively of all nutrients. In Haryana 662679, 17178 and 3950 Mg/year of N, P and K were used during 2001-2002, whereas the potential of sewage is worked out to be 8140, 121 and 11046 Mg/year. Thus approximately 2.4% of total micronutrients used can be supplied through sewage irrigation.

Utilization of sewage water through drip irrigation

Water shortage associated with intensive depletion of underground aquifers has prompted the search for alternative water sources. It has led to secondary treated domestic wastewater being considered for irrigation of field crop and raw eaten vegetable crops (Oron et al., 1991). Secondary domestic wastewater is now being used on a relatively large scale, mostly in developed countries, for field crops and landscape irrigation, groundwater recharge, and storage in recreational centers. In a few cases, tertiary or advanced treatment of the wastewater is required (Kirkpatrick and Asano, 1986). The concept of water saving might seem to contradict the idea of maintaining maximum yield from irrigated crops. The conflict might be more significant in arid zones with limited natural, high quality, permanent water source. A possible remedy to this conflict is to use non-conventional water, such as domestic treated wastewater, applied by drip irrigation. Using a subsurface drip irrigation system can further increase the efficiency of water application (Phene et al.,1985). The other advantages of use of drip irrigation system with sewage water are that no aerosols are formed, water logging due to runoff and deep percolation is negligible and the only contact with the water occurs when the product to be consumed touches the soil; the product of the plants growing above the soil being practically devoid of pathogens when the drip system is buried in the soil or covered by the plastic sheets (Capra and Scicolone, 2004).

Current scenario of sewage water utilization in India

Currently, about 30% of untreated sewage water is being utilized to grow vegetable crops around urban center using surface method of irrigation. This practice, besides a health risk to the farmers and the consumers of the product, it is causing enormous ground water contamination since excessive deep percolation losses cannot be avoided. Moreover the productivity of land and water is quite less, which could be increased substantially by adapting drip irrigation. Around 60 % of sewage water is directly disposed off in surface water bodies and low lying areas causing groundwater and surface water contamination and inefficient use of our water resources. Visualizing the alarming level of environmental pollution, around 10 % of sewage water is being treated in conventional sewage treatment plants generating mostly primary treated sewage water. These waters are also utilized for irrigation purpose with surface method of irrigation.

Bioremediation of wastewater

Use of wastewater in agriculture has been increased in recent years. However, wastewater, particularly from industries contains high concentration of heavy metals which enter into human beings and animals through food chain. Therefore, before its use in agriculture, it is desirable to remove these heavy metals from wastewater through low cost technology like efficient microbial culture. Biomass of microbes acts as adsorbent to remove heavy metals from wastewater. The ability to remove heavy metals from wastewater varies greatly among microbes. This needs to be exploited for removal of heavy metals from wastewater through efficient microbes. Laboratory experiments conducted at CSSRI, Karnal for bioremediation of heavy metals through microorganisms showed encouraging results. Removal of lead and cadmium from liquid medium by fungi cultures (Aspergillus awamorii, Trichoderma viride and P. chryosporium) was found to be substantial. The above results indicate potential of some of the fungi for removal of heavy metals like Pb and Cd which can be used for bio-treatment of wastewater at low cost and in eco-friendly way.

Epilogue

Disposal of the wastewater is a serious problem especially in developing countries, which is causing groundwater and surface water contamination and creating environmental pollution. The conventional method of disposal of sewage water (sewage treatment plants) are cost intensive and beyond the reach of many municipalities. The use of sewage water through subsurface drip irrigation may help to solve the disposal problem and finding a solution, which may be economically viable. Though its adaptation for efficient utilization of good quality and saline water is steadily increasing in India still there are few challenges. Investment needs being high, the technology could be popularized through one window system of financial assistance including subsidy. Its large-scale expansion will reduce many of its shortcomings, which will also encourage the use of sewage water through drip irrigation. Wastewaters become more dangerous and even unsuitable when contaminated with significant loads of industrial effluent. Some of the low cost technologies which reduced the crop contact with wastewater like planting on raised beds, washing the produce with clean water and discarding the most contaminated part resulted in significant reduction of pathogen load. Viable options include disposal of wastewater in tree plantations and phyto-remediation through flowers and aromatic plants where produce is non-edible but economical.

References

Capra, A. and B. Scicolone, 2004. Emitters and filter tests for wastewater reuse by drip irrigation. Agricultural Water Management, 68, 135-149.

Kirkham, W.R.,and Asano,T.1986. Evaluation of tertiary treatment system for wastewater reclamation and reuse. Water Sci. Tech., 18 (10), 83-95.

Oron, Gideon, Joel Demalach, Zafrir Hoffman and Rodica Cabotaru, 1991. Subsurface microirrigation with effluents. Journal of Irrigation and Drainage Engineering, 117(1), 25-37.

Phene, C.J., Hutmacher, R.B.., Davis, K.R.,and McCormic, R.L.1985. Subsurface drip irrigation offers success. Calif.-Ariz. Farm press, 7/8 (40), 24-31.