Occurrence and fate of Ascaris lumbricoides ova in biosolids in Victoria, Australia: a human health risk assessment of biosolids storage periods.

Reuse of sewage biosolids in Victoria, Australia, typically involves mesophilic anaerobic digestion followed by air-drying and long-term storage to ensure removal of ova of soil-transmitted helminths (STH) such as Ascaris lumbricoides. Long-term storage degrades the biosolids' agronomic quality due to the loss of key plant nutrients and takes up large areas of storage space. The impact of varying biosolids holding times and other processes on STH using Ascaris as the reference STH pathogen was examined in this study using a quantitative risk analysis approach. Risk modelling of the potential human health impacts from the presence of Ascaris ova in biosolids was undertaken for discrete holding periods of 1, 2 and 3 years. Modelling showed that to meet the WHO 1 µDALY·person-1·year-1 disease burdens guideline for limiting exposure category, a biosolids storage period of 1.24 years or 2.1 years would be required, depending on the data source of ova shedding rates per worm (Bangladesh or Nigeria, respectively). The soil exposure and salad/root vegetable consumption models included a number of variables with moderate to high degrees of uncertainty. Monte Carlo simulation was used to assess the effect of uncertainty in model input variables and to assist in highlighting areas for further research.

Helminth log reduction values for recycling water from sewage for the protection of human and stock health.

The LRVs required to decrease HE concentrations in raw sewage to an acceptable level to manage the risk to human and livestock health were determined. An LRV of 3.0 was required to meet the HBT of 1 µDALY pppy in SE Australia where human helminth infections are not endemic. In comparison, a similar exposure volume and LRV in endemic regions would result in a HBT of 100 µDALY pppy. The risks posed by cattle- and pig-related helminths were also managed acceptably with the treatment of sewage providing an LRV of 3.0. New design equations were derived to determine LRVs based on hydraulic residence times (HRTs) in an activated sludge plant (ASP) and lagoons. The new equation for lagoons indicated that an LRV of 3.0 could be achieved with a HRT of 18 days or less.

Accelerated seeded precipitation pre-treatment of municipal wastewater to reduce scaling.

Membrane based treatment processes are very effective in removing salt from wastewater, but are hindered by calcium scale deposit formation. This study investigates the feasibility of removing calcium from treated sewage wastewater using accelerated seeded precipitation. The rate of calcium removal was measured during bench scale batch mode seeded precipitation experiments at pH 9.5 using various quantities of calcium carbonate as seed material. The results indicate that accelerated seeded precipitation may be a feasible option for the decrease of calcium in reverse osmosis concentrate streams during the desalination of treated sewage wastewater for irrigation purposes, promising decreased incidence of scaling and the option to control the sodium adsorption ratio and nutritional properties of the desalted water. It was found that accelerated seeded precipitation of calcium from treated sewage wastewater was largely ineffective if carried out without pre-treatment of the wastewater. Evidence was presented that suggests that phosphate may be a major interfering substance for the seeded precipitation of calcium from this type of wastewater. A pH adjustment to 9.5 followed by a 1-h equilibration period was found to be an effective pre-treatment for the removal of interferences. Calcium carbonate seed addition at 10 g l(-1) to wastewater that had been pre-treated in this way was found to result in calcium precipitation from supersaturated level at 60 mg l(-1) to saturated level at 5 mg l(-1). Approximately 90% reduction of the calcium level occurred 5 min after seed addition. A further 10% reduction was achieved 30 min after seed addition.

Soil factors controlling the toxicity of copper and zinc to microbial processes in Australian soils.

Abstract-Two soil microbial processes, substrate-induced nitrification (SIN) and substrate-induced respiration (SIR), were measured in the topsoils of 12 Australian field trials that were amended separately with increasing concentrations of ZnSO4 or CuSO4. The median effect concentration (EC50) values for Zn and Cu based on total metal concentrations varied between 107 and 8,298 mg kg(-1) for Zn and 108 and 2,155 mg kg(-1) Cu among soils. The differences in both Zn and Cu toxicity across the 12 soils were not explained by either the soil solution metal concentrations or CaCl2-extractable metal concentrations, because the variation in the EC50 values was larger than those using total concentrations. Toxicity of Zn and Cu decreased with increasing soil pH for SIN. For Cu, also increasing cation exchange capacity (CEC) and percent clay decreased the toxicity towards SIN. In contrast to SIN, soil pH had no significant effect on toxicity values of SIR. Significant relationships were found between the EC50 values for SIR and background Zn and CEC for Zn, and percent clay and log CEC for Cu. Relationships such as those developed in this study will permit Australian environmental regulation to move from single-value national soil quality guidelines to soil-specific quality guidelines and permit soil-specific risk assessments to be undertaken.

Do earthworms mobilize fixed zinc from ingested soil?

A wide range of organisms inhabit the soil and has to deal with soil-bound metals. The bioavailable fraction of metals may be estimated explicitly using the isotopic dilution technique. In the present paper, we evaluated the isotopic exchange technique for assessing the bioavailability of soil Zn (using 65Zn) to earthworms. To validate the technique, the worms were first exposed to various 65Zn levels, and errors due to soil entrained in the gut were evaluated. This exposure indicated no effect of gamma-radiation on growth (wet weight gain) of the organisms and that depuration of the earthworms minimized errors in labile pools determined by isotopic dilution. Our study further showed that the earthworms accessed 55-65% of the total Zn in the soil. The labile pool for the earthworms Eisenia andrei was similar to that for the plant Lactuca sativa, indicating that earthworms and plants to a large extent access the same fraction of soil Zn. Hence, the isotopic dilution technique has the potential to assess biologically available pool of Zn in soils. As lettuce is not known to significantly mobilize nonlabile metals in soil, this study indicates that Zn uptake by E. andrei is predominantly via the exchangeable pools (possibly the soil pore water) rather than dissolution of Zn held within soil particles or within soil organic matter or other food sources.

Adaptation of soil biological nitrification to heavy metals.

The adaptive response of soil biological nitrification to Zn and Pb was assessed using an in situ method we have developed. The method is based on reinoculating a sterilized metal contaminated soil with the same soil that is either uncontaminated or has been incubated with metal. This approach excludes the potentially confounding effects of metal aging reactions in soils. We found added Zn concentrations which gave rise to a decrease in nitrification to 50% that of the uncontaminated soil (i.e. EC50) of 210 mg/kg for communities not previously exposed to Zn and 850 mg/kg for communities exposed to Zn for 17 months, indicating that significant adaptation of the community to Zn had occurred. Similarly, this protocol was able to demonstrate adaptation of soil biological nitrification to Pb, with EC50 values of 1960 and 3150 mg/kg for the unexposed and exposed treatments, respectively. Exposure of unadapted and adapted microbial communities to a combination of Zn and Cd showed that the presence of Cd did not lead to greater toxicity in either community. Adapted communities were not more sensitive to decreases in soil pH than unadapted communities. Prior exposure to Zn was found to confer significantly greater tolerance of the community to Pb. Prior exposure to Pb similarly conferred significantly greater tolerance of the community to Zn. Implications of the adaptive capacity of soil microbes to the development of critical threshold values for heavy metals in soil based on ecotoxicity assessments are discussed.

Determining toxicity of lead and zinc runoff in soils: salinity effects on metal partitioning and on phytotoxicity.

When assessing cationic metal toxicity in soils, metals are often added to soil as the chloride, nitrate, or sulfate salts. In many studies, the effects of these anions are ignored or discounted; rarely are appropriate controls included. This study used five soils varying in pH, clay content, and organic matter to determine whether salinity from counter-ions contributed to or confounded metal phytotoxicity. Varying rates of Pb and Zn were applied to soils with or without a leaching treatment to remove the metal counter-ion (NO3-). Lactuca sativa (lettuce) plants were grown in metal-treated soils, and plant dry weights were used to determine median effective concentrations where there was a 50% reduction in yield (EC50s) on the basis of total metals measured in the soil after harvest. In two of the five soils, leaching increased the EC50s significantly for Zn by 1.4- to 3.7-fold. In three of the five soils, leaching increased the EC50s significantly for Pb by 1.6- to 3.0-fold. The shift in EC50s was not a direct result of toxicity of the nitrate ion but was an indirect effect of the salinity increasing metal concentrations in soil solution and increasing its bioavailability for a given total metal concentration. In addition, calculation of potential salinity changes in toxicological studies from the addition of metals exhibiting strong sorption to soil suggested that if the anion associated with the metal is not leached from the soil, direct salinity responses could also lead to significant overestimation of the EC50 for those metals. These findings question the relevance of the application of single-metal salts to soils as a method of assessing metal phytotoxicity when, in many cases in our environment, Zn and Pb accumulate in soil over a period of time and the associated counter-ions are commonly removed from the soil during the accumulation process (e.g.. roof and galvanized tower runoff).