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

Abstract

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<0.05) with sludge treatment. AeD sludge had significantly higher TN and TC than AnD sludges per unit mass sludge. Cumulative CO2 – C evolution was significantly higher in AeD compared to AnD sludges. Net N mineralized per unit mass sludge was also higher in AeD, but the anaerobic sludges had higher N mineralization rates percentage, recording a final rate of 43% and 54% for AnDP0 and AnDP1 respectively against 41% for AeD by the end of incubation period. Thus, the greater amount of net N mineralized was a result of higher applied organic N in AeD relative to AnD sludges rather than a faster mineralization rate. Addition of cationic polymeric material appeared to increase net N mineralization rate, registering ±10% higher in AnDP1 above the AnDP0, although the difference was not statistically significant. The findings revealed that, larger fractions of N in sludges are likely to be mineralized faster within the earlier days of application, suggesting that for efficient utilization of mineral N from such sludge application, planting should be planned to synchronize crop N needs with the high N release period of biosolids which is ±30 days of application. At the same time, one anaerobically digested sludge (AnDP1) dried at 25 cm depth was selected for a field N mineralization study at six sites under different climatic conditions. A field incubation study was conducted from October 2017 to June 2018 (273 days) on six sites representing five South African agro-ecological zones. Porous ceramic cups filled with soil-biosolids mix (treatment) and soil alone (control) were used for the study. The cups were buried at 15 cm depth below the soil surface. Net nitrogen mineralization from the cups was estimated as the difference between initial and final organic N applied. Net N mineralization rate varied significantly among agro-ecological zones. Lowest mineralization rate was recorded in arid zone (10%) and the highest in super-humid zone of Nelspruit (57%). Net N mineralization rate varied with agroecological zone and was 10% (arid), 31% (semi-arid), 34% (sub-humid-Bethlehem), 46% (sub-humid-Johannesburg), 48% (humid), and 57% (super-humid). It was apparent from the field incubation study that a large proportion of the mineralized N was lost from the porous cups. Overall, the findings of the study suggest that a single nationwide biosolids application rate is unsustainable and would not suitably meet the crop N requirements across varied agro-ecological zones. Understanding the long-term effects of sludge application on the physical, chemical and biological soil parameters remains a major ecological endeavour. These parameters may be useful indicators on the effects of long-term treatments on soil quality. Soil samples from a fourteen years long-term experiment were evaluated for sludge effects on soil microbial properties. The samples were collected, and an assessment was conducted on microbial community structure, biomass and extracellular enzymatic activities in dryland maize, irrigated maize and grassland soils under repeated application of sludge and or synthetic fertilizers. It was shown that these soils are relatively stable with no substantial variations in major and minor nutrients. However, pH showed significant (P<0.05) correlation with several taxonomic groups and was a major driver of the community structure. An inverse correlation between microbial biomass nitrogen (MBN) and sludge application rate in irrigated maize and grassland systems was observed where MBN decreased with increased sludge application. While microbial biomass carbon (MBC) significantly (P<0.05) increased with fertilizer and sludge application, there was no significant difference between sludge application rates (P>0.05). The relative stability in community structure and composition was also reflected in the extracellular enzymatic activities recorded across treatments, in which no significant differences were observed. Taxonomic analyses showed that Proteobacteria, Acidobacteria, Actinobacteria and Verrucomicrobia were dominant across treatments although their relative abundances and diversity were not significantly different across treatments. Partial Least Square Structural Equation Modelling (PLS-SEM) showed that soil chemistry was a major driver of microbial abundance but not extracellular enzymatic activities. In turn, microbial abundances showed strong and significant relationship with both diversity and extracellular enzymatic activities. These findings suggest that long-term sludge and fertilizer amendments in agricultural lands would likely stabilize the soil systems resulting in resilience of soil microbiota. The study indeed demonstrated the importance of critical analyses and understanding the sludge material one is dealing with for agricultural purposes, as its characteristics, use suitability, dynamics and behaviour in the field is brought about by a chain of practices and at different stages of sludge production.