Wednesday, 30 August 2017

Impact of irrigation with industrial treated wastewater on soil properties

In large cities of developing countries, food demand is partially supplied by urban agriculture performed by smallholders involved in market-oriented farming. Although considered as an informal economy, urban agriculture generates significant incomes for the underprivileged part of the population, i.e. women and unemployed young people. However, urban farming is highly dependent on water availability.



Water availability is the main limiting factor for urban agriculture in arid and semi-arid regions. Such a problem is mostly seen in countries like sub-Saharan Africa where priority is given to industrial wastewater consumption. Large amounts of wastewater from household and industrial activities are discharged in water channels. Such effluents are rarely subject to suitable treatment and so they remain the only sustainable water resource available for farmers. There is no doubt that irrigation with wastewater increases the yield in agriculture as compared to irrigation with fresh water. This actually happens due to supply of essential nutrients like N, P & K present in wastewater.

Wastewater also contains heavy metals in greater amounts, so that it does not meet the quality criteria for irrigation water and imparts adverse effects on soil properties and quality. Main adverse impacts are soil salinization, sodication/alkalinization and structural changes which may result in dramatic yield increase. Soil sodication is associated with rise in soil pH which causes N, P, K & Zn deficiencies and formation of black alkali on surface of soil due to dissolution of organic matter. It may also result in clay dispersion. The black alkali is an indicator of soil alkalinization and it shows irreversible degradation of soil because organic matter is lost resulting in depletion of soil fertility.

According to Abbott and Hasnip (1997), Water with RAcalcite > 2.5 mmolcl-1 and EC < 4000┬ÁS cm-1 values (alkali water) may pose a threat to soil structure due to high amount of sodium. Irrigation with such wastewater changes the soil properties. Sodication induces dispersion of clay particles and a collapse of structural pores which causes obstruction of pores in subsurface horizon. Irrigation with wastewater also results in increase of bulk density and dramatic decrease in structural porosity.

Other causes, such as biological clogging due to the production of poly-saccharide and other organic compounds or physical clogging by suspended solids may be discarded in our situation, considering the relative small values of total suspended solids (205±157 mg l-1) and biological oxygen demand (107±50 mg l-1) of the effluent. Up to now, most studies about wastewater impacts on soil were limited to properties such as infiltration rate and hydraulic conductivity. The present study confirms a risk of strong degradation of hydro-structural properties when a soil is irrigated with alkaline and sodic wastewater, resulting in a dramatic decrease of soil fertility.



Conclusion:

Highly sodic and alkaline industrial effluents stemming from a microphyte sewage treatment plant used for irrigation led to quick soil structure collapse. As a result, dark deposits appeared at the soil surface of soils irrigated with wastewater. Chemical analysis clearly showed sodium and bicarbonates accumulation and pH increase to alkaline values in the topsoil of soil irrigated with wastewater. Physical analysis exhibited structural pore network of the soil collapsed dramatically, resulting in compact layers with poor water storage. Dissolution of organic matter leads to formation of black alkali at soil surface. It results in 50% decrease in yield.





About Author:
Miss Bushra Niamat
Institute of Soil & Environmental Sciences,
University of Agriculture, Faisalabad-38040, Pakistan

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