Properties and potential uses of water treatment sludge from the Neches River of southeast Texas

Abstract

Land application of water treatment plant (WTP) sludge has been an unsolved problem. The objectives of this study were (1) to investigate characteristics of organic polymer sludge, and (2) to determine the effects of the sludge on soil properties that influence utilization of the sludge as a soil amendment. Water treatment sludges were obtained from water utilities along the Neches Rivet-near Beaumont, Texas. They were mostly coagulated with organic polymers. Mineralogical composition, cation exchange capacity (CEC), scanning and transmission electron microscopy (SEM and TEM), aggregate stability, Atterberg limits, hydraulic conductivity, dispersion, crust strength, adsorption characteristics, nitrogen content and mineralization potential of the sludge or sludge-amended soil were determined in this study. Mineralogical composition of organic polymer sludge was similar to local Beaumont clay soil. The major fraction of the sludge, the coarse clay, was estimated to be 40% kaolinite, 32% smectite, 20% quartz and 8% mica. However, XRD patterns of the sludge indicated that expansion of the smectite was inhibited by organic polymer coagulant added during water clarification, which also was confirmed by aggregate stability of the sludge. Cation exchange capacity of the bulk sludge samples ranged from 8 to 28 cmol kg-1. Quantitative mineralogical analyses showed that CEC of the sludge was reduced by blockage of cation exchange sites with organic polymers. SEM and TEM results indicated that the sludge was mostly fine aggregates of clay particles. Dried sludge aggregates were not prone to swell, due to their resistance to rewetting. The aggregate stability of dried sludge was above 90% after a 24 hours soaking period in water, compared to 7% aggregate stability of local Beaumont clay soil. Addition of 0 to 10% sludge to Boonville sandy loam soil increased the aggregate stability of the soil from 4 to 13%. Atterberg limit showed that wet sludge had wide ranges of moisture contents in semi-solid and plastic states, and shrunk greatly during drying. Shrinkage limit of the sludges ranged from 16 to 66%; plastic limit from 111 to 138%; and liquid limit from 208 to 320%, which suggested that the sludge was highly plastic and compressible. Addition of 0 to 10% sludge into Boonville sandy loam soil increased the infiltration rate of the soil two orders of magnitudes from 1.4 x 10-4 to 1. I X 10-2 CM / S, and reduced the dispersion of the soil significantly. Moreover, addition of from 0 to 10% sludge into Boonville sandy loam soil greatly reduced the crusting produced by rainfall, and the penetration resistance of the crust decreased from 53.1 to 14.4 kg / cm. Metal adsorbing ability of the Boonville sandy loam soil was reduced by addition of the sludge. Adsorption of Zn+2 decreased from 19.7 to 17.7 ug / g when the sludge was amended from 0 to 10%. Total nitrogen and exchangeable ammonium nitrogen (NH4+) contents of the sludge were four times and twenty times as high, respectively, as those of local Beaumont clay and Lake Charles clay soils. Organic polymer coagulants added during water treatment apparently increased nitrogen content of the sludge. Although the sludge contained considerable total nitrogen, incubation experiment of sludge-amended Beaumont clay or Lake Charles clay soil showed that nitrogen mineralization rate was not affected by the sludge addition. This study indicated that the sludge can improve soil physical properties significantly, e.g. soil aggregation, infiltration, dispersion and crusting. The low nitrogen mineralization rate of the sludge showed that the sludge had little fertility and would not produce a groundwater contamination problem.