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Quantifying the fertilizer value of wastewater sludges for Agriculture
Expanded Title:Beneficial use of treated municipal sludge in agricultural lands is a well-known practice around the world, because of its soil conditioning effect and as a source of low grade fertilizer. Municipal sludge contains appreciable amounts of N and P and has a significant inorganic fertilizer replacement value for these two major plant nutrients. In addition, it is a good source of Ca and Mg and essential plant micro-nutrients, including Zn, Fe, Cu, Mo, Mn, and B. Beneficial agricultural use of sludge accounts for 28% of the total sludge produced from South African wastewater treatment plants. In order to accommodate the complex interactions between sludge, soil, climate and cropping systems, a mechanistic N and P balance cropping systems model (SWB-Sci) model was developed as a reasoning support tool. This model is a fairly complex scientific research tool and requires detailed weather, soil, crop, and sludge parameters. Due to its complexity, routine use of this reasoning support tool by industry or extension officers in its current form is highly unlikely. One of the main concerns with the beneficial use of sludge in agricultural lands is the longterm build-up of trace metals. Some trace metals are essential for plant and animal health while others are not known to be essential to plants and animals. Unfortunately there is currently no information available on the effect of soil water availability under either rainfed or irrigated systems on the dynamics of metals in the soil-plant system from class A1a sludge amended soils. In addition, the unwritten rule of using environmental soil contamination threshold values as phyto-toxicity indicators needs to be investigated. The key findings of the study were: 3 Simple user-friendly database model development A single annual N release rate could not be used across sites within an agro-ecological zone. Using the SWB-Sci model and 20 years of measured weather data for five of the six South African agro-ecological zones, model simulations showed that: 1. Annual N mineralization rates varied significantly (P<0.05) across agroecological zones ranging from 25% in arid to 42% in super-humid agroecological zones. Similarly, annual N mineralization rates varied significantly between sites within an agro-ecological zone. 2. Annual N mineralization rates did not differ significantly (P<0.05) between seasons within an agro-ecological zone. 3. Annual N mineralization varied significantly between soil textures within a site. Therefore, annual N mineralization rates within a site should be adjusted for each soil texture. 3 Characterization of sludge N and P pools Drying sludge in thick layers of more than 250 mm for longer duration periods of more than 20 days seems to decrease the sludge N and C content. There was, however, little or no difference in total N concentration between sludges treated with anaerobic digestion and activated sludge when both were exposed to similar drying techniques as related to (liquid sludge depth in drying beds and drying duration). Large fractions of the organic matter in all sludges types were in the form of water-soluble compounds. The lignified fraction was relatively higher for the sludges that were dried in thick layers for longer time regardless of whether dried in concrete beds or paddies. Sludges dried in thin layers within a shorter period of time (7 days) were characterized by higher N, C, and low lignin contents and were dominated by soluble compounds and hemicellulose. Therefore, further investigation on the effect of drying depth and time using similar sludge sources and treatment processes, is required to isolate the effect of sludge source. Similar to previous findings, sludge C and N decomposition was significantly influenced by the initial N concentration, C:N ratio, lignin content, and Lignin:N ratio of the sludge. C and N mineralization per unit organic C applied was highest for activated sludge and lowest for anaerobically digested paddy dried sludge. Considering the higher N content of activated (ACTIVE) sludge and higher N release rate per unit organic N applied, the fertilizer value of activated sludge was higher than the other sludge types investigated. The study also showed that about 90% of the total C decomposition came from the soluble organic carbon fraction, indicating that the soluble organic fraction is the main source of C and N decomposition in municipal sludge. The next main contributor was found to be hemicellulose, which accounted for 5-7% of the total C decomposition. 3. Simple heavy metal mass balance development Field plots were arranged in a randomized complete block design comprising four replications of three treatments (0, 8, and 16 Mg ha-1yr-1 anaerobically digested municipal sludge) planted to dryland maize and irrigated maize-oat rotation. Soil and plant samples were collected following 7 years of treatment application for selected metal analyses. Crop uptake and accumulation in the soil profile of Zn, Cd, Ni and Pb increased as the sludge application rate increased. Concentrations in tissues of the test crop remained well below phytotoxic levels, except for Zn under dryland maize production that received sludge at 16 Mg ha-1 yr-1. Concentrations of the selected pollutants in the soil profile of all sludge treatments remained below threshold levels as stipulated in the South African sludge guideline as well as international guidelines. A large fraction of these metals was EDTA extractable. The saturated paste extractable fractions of Cd and Pb were <1 mg kg-1. The water soluble fractions of Zn and Ni were significantly higher than that of Cd and Pb but accounted for <1% of the EDTA extractable fractions, indicating the mobile nature of these elements and potential for leaching and groundwater contamination. Therefore, it is of utmost importance to have integrated sludge management practices and rigorous heavy metal monitoring protocols below the top 0.3 m plough layer for sustainable beneficial agricultural use of sludge.
Date Published:01/07/2015
Document Type:Research Report
Document Subjects:Water Resource Management/IWRM - Planning and development, R & D - Methodology
Document Format:Report
Document File Type:pdf
Research Report Type:Standard
WRC Report No:2131/1/15
ISBN No:978-1-4312-0691-9
Authors:Tesfamariam EH; Annandale JG; de Jager PC; Ogbazghi Z; Malobane ME; Mbetse CKA
Project No:K5/2131
Originator:WRC
Organizations:University of Pretoria
Document Size:1 682 KB
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