Recycling municipal wastewater sludge in agricultural land : implication on plant nutrient supply and biological indicators of soil quality

World-wide population is rising, and pressure is mounting on already limited resources targeted for agricultural productivity. Resources like water and crop nutrient sources are getting limited every day to sustain the growing population. At the same time, production of municipal wastewater sludge is increasing, being exacerbated by urbanization and industrialization. When wastewater sludge is handled as a waste rather than as a resource, there are high costs associated with transportation and storage, as well as costs to the environment. As such, this has compounded pressure on strained financial and land resources needed for managing this product. Critical management strategies are needed for sustainable utilization of wastewater sludge. Therefore, it is critical to create management practices that would facilitate reuse options of this product without polluting the environment, and at limited costs. Sludge recycling in agricultural lands was observed to be the best sustainable option as it would bring multiple benefits in doing so. Therefore, to better understand the sustainable options in sludge recycling in agricultural lands, survey, field, and laboratory studies were conducted. The aim of the study was to explore the general value /production chain of municipal wastewater and sludge materials, characterize them and quantify their implication to soil chemical and biological properties in agroecosystems. A survey involving eighteen liquid municipal wastewater sludges was conducted. From the agricultural use suitability assessment and characterization of the sludges, the results revealed that heavy metals concentrations were below the threshold level except for two waste activated sludge (WAS) which were downgraded to class B due to high lead (Pb) content. Two anaerobically digested (AnD) sludges were downgraded to classes B and C because of high Pb and cadmium (Cd) concentrations respectively. Electrical conductivity (EC) was above the 200 mS m-1 threshold in AnD and in one of aerobically digested (AeD) sludges whilst WAS had ≤100 mS m-1. Sustainable sludge application options observed were either to apply dry sludge based on crop N or P requirements and supplement the nutrient deficit with other fertilizer sources. The use of current liquid sludge as irrigation water to meet crop water needs and or applying dry sludge at 10 tons ha-1 options showed that such options are unsustainable and would add excess nutrients above crop need which would result in agroecosystems pollution. After characterization, a single WWTP was selected from which three sludge types based on their treatment processes were dried in sand drying beds at five drying depths/thicknesses. Anaerobic digested sludge without polymer (AnDP0) and with polymer (AnDP1) and aerobic digested sludge (AeD) were used in this study. It took 63 days in winter and 45 days in spring for deepest depth sludge drying to the target moisture content of 10%. The measured N fractions and other parameters were more influenced by sludge types than drying depth. TN content followed the order of AeD>AnDP1>AnDP0. No consistent trend with depth was seen. Inorganic N significantly increased with drying for AnD sludges. Inorganic N also increased with drying depth largely in winter relative to spring drying. Drying depth affected more sludge parameters in winter than spring which could be attributed to longer drying time experienced in winter. Polymeric material addition to sludge had little effect on initial properties or during drying. Its major effect was on length of sludge drying times and land size requirement. Sludges dried faster in shallower than deeper depths and in fewer days for AeD and AnDP1 than AnDP0. Drying sludge at 15 cm was the best option across sludge types in winter taking an average land area between 261 m2 and 383 m2 serving a one hundred thousand size population in the fewest sludge drying days, whilst in spring, the most favourable option was drying at 20 cm depth. The study findings suggest drying bed management can be based on land area requirements with little concern for biosolids quality changes. For WWTPs that rely on drying beds, the use of polymeric materials is not much of importance because it will be a non-crucial additional cost. After drying in beds, samples of the three sludge types were taken for a ninety-day laboratory incubation study for N and C mineralization. The results revealed that the measured parameters like pH, EC, TP and N varied significantly (P