Abstract

Intensive farming generally needs large addition of organic matter to maintain fertility and enhance crop yields. Sewage sludge/biosolids are by-products of municipal and industrial wastewater treatment and a rich source of organic nutrients. Sewage sludge having high content of organic matter, macro- and micro-nutrients, can be used as fertilizer/soil conditioner for food, vegetable crop, horticultural plants and pasture, which in most cases can be beneficially recycled. In the past sewage sludge was regarded as a waste product due to expected high level of contaminants such as pathogens, pollutants and synthetic materials discharged in sewer from homes and industries, which were often incinerated, dumped in occasion or land fill. As a result of rapidly increasing population, urbanization and industrialization, wastewater production and sewage sludge generation have increased manifold. Due to high cost of mineral fertilizers and escalating trends in their prices, there is an increasing trend of using sewage sludge in agriculture, especially under intensive cropping in arid and semi arid regions of the country. Therefore, application of sewage sludge to agricultural soils may be sustainable and economical due to nutrient cycling and disposal of sewage sludge. However, there may be a risk in use of sewage sludge due to potentially harmful contents present in the sludge such as heavy metals and pathogens. This paper, therefore, presents a review on various aspects of sewage sludge used in agriculture.

Keywords

Sewage Sludge; Sustainable Agriculture; Pathogen; Heavy Metals; Soil Quality and Fertility

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1. Introduction

Sewage sludge consists of by-products of wastewater treatment. It is a mixture of water, inorganic and organic materials removed from wastewater coming from various sources (domestic sewage, industries), storm water runoff from roads and other paved area, through physical, biological, and/or chemical treatments. Sewage sludge is also referred to as biosolids. Various agencies have put forth different definitions of Biosolids. Biosolids are nutrients rich organic materials resulting from the treatment of domestic sewage in a treatment facility [1]. Some regards biosolids as stabilized organic solids derived from biological wastewater treatment process, which can be managed safely to utilize their nutrients on sustainable basis, and used for soil conditioning, energy, or other value [2]. The utilization of biosolids/sewage sludge in agriculture is gaining popularity as a source of waste disposal. Biosolids/sewage sludge generally contain useful compounds of potential environmental value. They also contain useful concentration of organic matter, nitrogen, phosphorus and potassium and to lesser extent, calcium, sulphur and magnesium. The availability of phosphorus content in the year of application to agricultural lands is above 50% and is independent of prior sludge treatment. Nitrogen availability is more dependent on sludge treatment. Untreated liquid sludge and dewatered treated sludge release nitrogen slowly with benefits to crop being realized over a relatively long period. Anaerobically digested sludge has high contents of ammonia, which is readily available to plants. Thus it is a nutrient enriched fertilizer. The organic matter content in sludge can improve soil physical, chemical, and biological properties with ensuring better cultivation and aquiferous capacity of soil [3], especially when applied in the form of dewatered sludge cake [4]. Biosolids reduce runoff and increase surface retention of rain water [5]. Organic nitrogen in sludge is much less likely to cause ground water pollution than chemical nitrogen fertilizer [6]. The application of sewage sludge to agricultural land is the best way of recycling the nutrients present in it. Therefore, sewage sludge may be considered an important biological resource for sustainable agriculture. It produces favorable plant yield responses, when used as an organic fertilizer [7,8].

The only drawback in the use of sewage sludge on agricultural land is its pollutant load (including heavy metals, organic compounds, pharmaceuticals and pathogens). Potentially toxic elements, is a general term for wide range of metals that originate in sewage, such as cadmium, copper, nickel, lead, zinc, mercury and chromium. The metals are concentrated in the sludge as a result of their association with settable solids during primary and secondary treatment process [9]. Research on land application of sewage sludge has mainly focused on its role in introduction of heavy metals in the food chain. However, it has been shown that sewage sludge application at recommended rates increased microbial activity in soil and tied up the heavy metals making them unavailable to plant and soil [10]. Another factor which needs careful attention regarding the use of sewage sludge/biosolids is prevalence of pathogens. It contains pathogenic bacteria, viruses and protozoa along with other parasitic helminthes, which can give rise potential health hazards to human, animals and plants. Salmonella and Taenia were identified as the greatest concern and risk to health from microbes in sewage sludge applied to land [11]. The number of pathogenic and parasitic organisms in sludge can be significantly reduced before application to land by appropriate sludge treatment and potential health hazard is further reduced by the effect of climate, soil microorganism and time after the sludge is applied to the soil [4].

Information on all these aspects in Pakistan and many under developed countries is limited and scanty. Therefore, this review paper focuses discussion on these important issues so that conclusions can be drawn as how for the sewage sludge is useful in sustaining soil quality and agricultural productivity.

2. Characterization of Sewage Sludge

There is an interesting trend in the agricultural application of sewage sludge obtained from wastewater treatment plant due to the possibility of recycling valuable components: organic matter, N, P and other plant nutriaents. Characterization of sewage sludge is extremely important prior to sludge disposal or application to farm land because there is a risk of toxic elements accumulation in soil [12,13]. Moreover, there is potential health hazard due to pathogens in the sewage sludge. The content of the sewage sludge will depend on the original pollution load of the treated water, treatment process applied to waste water and sludge [14]. Sludge treatment affects its composition. Examples of effective sludge treatment processes are presented in Table 1 and various types of sludge used in European countries are given in Table 2.

2.1. Fertility Parameters of Sewage Sludge

2.1.1. Organic Matter and Major Nutrients

Organic matter content and major nutrients were found to be quite high in sludge (Table 3). These are the parameters which are considered essential for enhancing soil fertility/quality and sustaining soil productivity.

2.1.2. Micronutrients and Heavy Metals

The issue of micro-nutrient and heavy metal concentration in sewage sludge needs careful attention for its use in agriculture. If the concentration of heavy metals is high in the sewage sludge then its continuous use year after year may result in toxic accumulation of heavy metals in the soil, which may have phytotoxic effects on various cereals, vegetables, fruits, pastures, and fodder crops. Consumption of these commodities by human beings and animals can cause health hazards to them. It is, therefore, essential to determine the heavy metal content of sewage sludge before its recommendation for use in agriculture.

Table 4 gives critical concentration of heavy metals in sewage sludge in different countries. Sewage sludge with heavy metal concentration higher than critical level for a given country is not permitted to be used as organic fertilizer for that country. In general, Germany is stricter than United States and European member states with regard to critical levels as shown by the lower values imposed by Germany. Heavy metal ranges in sewage sludges of European member states (Table 4) are comparatively lower than critical level adopted by them. In India the heavy metal ranges were higher than ranges in the European member states (Table 5). However, these values were either lower or closer to those mentioned in Table 4.

Conflicts of Interest

The authors declare no conflicts of interest.

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