Select chemical and engineering properties of wastewater biosolids
Highlights
► Geotechnical, chemical and environmental properties of wastewater biosolids are examined. ► Results were discussed to seek suitability of aged biosolids as structural fill material. ► Management techniques are provided to minimize possible risks.
Introduction
Biosolids refers to dried sludge having the characteristics of a solid typically containing 50–70% by weight of oven dried solids. Sludge refers to solid–water mixture pumped from wastewater treatment lagoons having the characteristics of a liquid or slurry typically containing 2–15% of oven dried solids. The quantity of the municipal biosolids produced annually in the world has increased dramatically over the decades. In the state of Victoria, Australia alone, more than 2 million tons of biosolids are currently stored in lagoons or in stockpiles with a further annual production of 66,700 tons per annum (DNRE, 2002).
The characteristics of the biosolids vary around the world since the properties of biosolids depend on factors such as the composition of the wastewater, method of treatment process (US EPA, 2005) and age of the biosolids. Even within a specific treatment plant, the characteristics of the biosolids can change from time to time because of variations in the incoming wastewater composition (US EPA, 2005).
After being placed in stockpiles, biosolids can be viewed as a geotechnical material comparable to non-consolidated cohesive soils with high organic contents (Lo et al., 2002). Research studies show that similar to other high organic content soils; unstabilized biosolids are generally associated with high compressibility (Disfani et al., 2009b), high rates of creep, and possible unsatisfactory strength characteristics (Suthagaran et al., 2008), which increases the risk of excessive settlements in case of their application as load bearing media (Santagata et al., 2008).
For many years, the beneficial use of biosolids has been hindered by public opposition which is the result of concerns about pollutants in the biosolids, risk of disease, and nuisance issues such as odors (US EPA, 2005). Concurrently the current knowledge of the geotechnical engineering properties of human waste biosolids is relatively limited or unknown. This has led to the unwelcome reality that wastewater biosolids still end up in landfills in many parts of the world.
To cover the knowledge gap on the engineering properties of wastewater biosolids with the focus on their geotechnical and chemical properties a research study was undertaken. Geotechnical laboratory experimentation including sieve and hydrometer analysis, Atterberg limits, compaction tests, organic content, California Bearing Ratio (CBR), hydraulic conductivity, 1-D consolidation, long term consolidation (creep) and shear tests were conducted on samples collected from biosolids stockpiles in Western Treatment Plant (WTP), Melbourne. The geotechnical characterization was followed by a set of chemical tests including heavy metals, dichloro diphenyl trichloroethane (DDT and derivatives), organochlorine pesticides, different forms of nitrogen, phosphorus, total organic carbon, Helminths, viruses, and Cryptosporidium and Giardia. Results were discussed to investigate the possibility of using biosolids in road embankments.
Section snippets
Review of past studies
While there are several studies on engineering properties of sewage sludge, only few are available on geotechnical properties of untreated wastewater biosolids. The engineering properties of sewage sludge have been studied in recent years in several countries such as UK, Hong Kong, USA, South Korea, Turkey, Spain and Singapore.
Asakura et al. (2009) investigated the geotechnical properties of pure sludge and also sludge in blends with other waste material in order to determine the allowable
Material and sampling
Sampling of the biosolids was carried out from the top of three existing biosolids stockpiles (aged more than 20 years) at the Western Treatment Plant (WTP) in Melbourne, Australia. Bulk biosolids samples were collected in large polythene bags which were sealed to maintain the in situ moisture content of the biosolids. During sampling, ASTM practice for sampling aggregates was followed and all necessary precautions were taken to capture a sample containing representative particle sizes and
Engineering characteristics
Select geotechnical engineering characteristics of biosolids are presented in Table 1. Six different biosolids samples (two from each stockpile) were tested and the average value or the results range is reported in Table 1.
The natural moisture content of biosolids was determined on the basis of the oven dry mass corresponding to a drying temperature of 50 °C instead of the standard drying temperature of 105 °C and is reported in Table 1. This procedure was employed in order to prevent charring of
Chemical and biological properties
Six biosolids samples (two from each stockpile) were tested for different types of heavy metals, different forms of nitrogen, phosphorus, Total organic carbon, DDT and derivatives and organochlorine pesticides. Among six different samples, some variations of results were observed. Test results were shown in the forms of average contaminants concentration value, standard deviation and Biosolids Contaminant Concentration (BCC) in Table 2.
Biosolids contaminants are classified as Grade C1 or Grade C
Discussion
Results of this study shows that if assessed as suitable for geotechnical reuse, biosolids may be used in construction applications such as in roadways, roadway embankments or building pads. This depends of the technical requirement of each specific application such as gradation curve, CBR value, compressibility potential etc.
In the state of Victoria, Australia, the local state road authority defines Type B fill as material which has a minimum assigned CBR of 2–5%, be free from organics and has
Conclusion
Biosolids samples obtained from a wastewater treatment plant in Melbourne, Australia were tested to investigate its geotechnical and environmental characteristics. The biosolids samples were found to be classified as OH according to USCS. The wastewater biosolids show high moisture content, liquid limit and plasticity indices comparable to common organic soils. The specific gravity of biosolids was found to be substantially lower than that of natural inorganic soils. Compaction tests results
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Address: Faculty of Engineering and Industrial Sciences (H38), Swinburne University of Technology, P.O. Box 218, Hawthorn, VIC 3122, Australia.
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Address: Coffey Geotechnics, 126 Trenerry Crescent, Abbotsford, VIC 3067, Australia.