Characterisation of priority contaminants in residential wastewater

[摘要] EXECUTIVE SUMMARYContext and purposeWastewater quality affects the lifetime of sewerage assets, the operation of wastewatertreatment plants, and the beneficial reuse of biosolids and effluent.Water conservation and reuse increase the need to understand and control elements inwastewater that impact biosolids reuse and effluent quality. Among these elements, waterauthorities have identified as priority parameters of interest: arsenic, antimony, boron,cadmium, copper, chromium, chloride, fluoride, iron, lead, mercury, molybdenum, nickel,nitrogen, phosphorus, selenium, sodium, tin, zinc, total dissolved solids and colour.Whilst monitoring of trade waste is a routine activity carried by water authorities, lessinformation is available on the contribution of domestic wastewater to contaminant loadsreaching wastewater treatment plants. The quality of residential wastewater is directlylinked to householder attitudes and water consumption patterns. A few studies have beencarried out on metal contaminants in wastewater in Australia mainly in the 1990s.Since that time, given the current climate of water restrictions and water demandmanagement, households have been urged to decrease their potable water consumption andreuse grey water. The impact of such measures on the concentration of priority contaminantsin wastewater is not yet known. Yet they could impact wastewater quality and flows, and itsbeneficial reuse.This research aims to increase understanding of the characteristics of wastewater generatedin a residential catchment subject to water restrictions. In particular it aims to describeinvestigations into how wastewater quality changes over time and how it compares withearlier studies in the literature.This report is part of the Smart Water Fund project Round 3 – Project 5 Household sourcesof priority contaminants in domestic wastewater. The overall project aims to understand theorigins of contaminants in domestic wastewater and to evaluate strategies for their reduction.Previous reports in the series have investigated the loads and sources of contaminants inhouseholds by characterising household products, wastewater discharge patterns and theoperation of common household appliances.The data collected in this report will be used to evaluate the relevance of the outputs from aBase case model to the current wastewater discharged in a Melbourne residentialdevelopment. The Base case model will serve as the basis for the evaluation of contaminantreduction strategies in a future report.Research methodA residential sub-catchment located in the South-East of Melbourne was monitored for flowand priority contaminants from the 24th February to the 27th March 2008.The catchment selected was an enclave of 697 households built after the 1970s. Thesewerage reticulation main is comprised mainly of vitrified clay and was subject to lowinfiltration.Monitoring was conducted at three sampling locations within the sub-catchment, sites 1, 2and 3, which received sewage from 7, 163 or 697 residential households, respectively.Monitoring was comprised of:Priority contaminants in residential wastewater Page v(a) Week 1: Evaluation of the typical flow and wastewater levels;(b) Week 2: Equipment commission and verification of operating conditions;(c) Week 2-3: Collection of daily composite samples for inorganics and physical –chemical parameters such as metals, salts, colour, etc.(d) Week 4: Evaluation of hourly profiles over a 24 hour period.In total, 11 and 12 diurnal composite samples were prepared for sites 2 and 3 for elementalanalysis and 8 days of hourly profiles for sites 1 and 2. Flow and electrical conductivity werecontinuously monitored over the entire period.Volumetric flow was monitored continuously in the sewer. Chemical composition wasdetermined using samples collected from the 3 locations within the catchment as hourlycomposites and either analysed as received or combined into a daily composite sample.Wastewater was analysed for flow rate, conductivity, colour, metals and ionic species(arsenic, antimony, boron, cadmium, copper, chromium, chloride, fluoride, iron, lead,mercury, molybdenum, nickel, nitrogen, phosphorus, potassium, selenium, sulphur, sodium,tin and zinc), and total dissolved solids using standard methods. Colour was characterisedas total and non-biodegradable colour.The data obtained characterised the quality and flow conditions of the catchment in thecurrent environment of 3a water restrictions under dry weather flow.This report outlines the process of catchment selection, the monitoring procedure and thefindings of the study.FindingsWastewater flow followed a typical diurnal curve with peak flows occurring from 5am to 8amand from 5pm to 8pm during the week. During weekends the morning peak flow occurred acouple of hours later in the day.Wastewater in the catchment had mean pH values and corresponding standard deviations of7.04 + 0.22, electrical conductivity of 870 + 59 S/cm and 77 Pt-Co units of colour. Colourcould be reduced on average by 37% following aerobic digestion, which is the basis for thetreatment process adopted at the Melbourne Eastern Treatment plant.The most significant increases in concentration from tap water to wastewater were verifiedfor nitrogen, phosphorus, potassium, sulphur, chloride and sodium. Their averageconcentrations in wastewater were respectively 384, 41, 33, 7, 6 and 3 times those found intap water.On the other hand, the concentrations of contaminants such as antimony, arsenic, cadmium,cobalt, mercury, selenium, tin and molybdenum were below the limits of detection of currentanalytical instrumentation. Lead was detected in 76% of the samples at low concentrationsand close to the detection limit (0.5g/L).Concentrations of boron, sodium and phosphorus were lower in this catchment andchromium, nickel and fluoride concentrations were higher than those reported in previousstudies (Connor and Wilkie 1995 and CWW 2007).Total dissolved solids was only 12% higher compared to studies conducted in Melbourne in1995, but lower by 30% compared to a 2006-2007 study conducted in the west of Melbourne(CWW 2007).The source of such discrepancy has not been ascertained, but it is hypothesized that eithergreater infiltration into sewer from groundwater took place in the Western study or thatPriority contaminants in residential wastewater Page vihousehold practices, e.g. diversion of washing machine grey water might be practiced in thiscatchment. Further investigation may be required.Concentrations of individual elements changed from one day to another and from week toweek. Part of the variability is attributed to the multitude of wastewater dischargers and alsoto the unpredictability of household wastewater quality.The day of the week had no significant affect on the concentration of priority contaminants inthe catchment. This is contrary to previous studies reported in the literature.The concentrations changed during each hour of the day. Concentrations were higher in thehours from 5am to midnight. However, the concentration and load distribution during thisperiod varied for each element.For nitrogen and phosphorus the highest concentrations were observed in the morning, with70% of the mass load of the contaminants entering the sewer in that period. These wereattributed to human excretion. Whilst the concentrations of aluminium, iron copper, sodium,sulphur and zinc were more evenly distributed during the day, which indicates that theirgeneration is likely to be associated with household activities. Variability was also observedacross the different sites within the catchment, which was expected given the number ofconnections for each sampling site: 7, 163 and 697 connections.Key findingsThe majority of the contaminant concentrations were within a similar range to valuesrecorded in earlier Melbourne studies (Connor and Wilkie 1995, CWW 2007).Arsenic, antimony, cadmium, cobalt, mercury, selenium, tin and molybdenum were below thelimits of detection, which is similar to results reported in the literature for earlier Melbournestudies.Lead was detected at concentrations close to the detection limit of 0.5g/L.Boron, sodium and phosphorus were detected at lower concentrations;Chromium, nickel and fluoride concentrations were detected at higher concentrations thanthose reported in the literature (Connor and Wilkie 1995 and CWW 2007).Total dissolved solids in this catchment was only 12% higher compared to studies conductedin 1995, but 30% lower compared to a 2006-2007 study conducted in the west of Melbourne(CWW 2007).RecommendationsThis report provides a snapshot of the quality of wastewater in a domestic catchment.To gain a greater understanding of the statistical significance of the data collected in relationto wastewater quality throughout the year, and to complete further investigations and toexplore if any correlation between priority contaminants could be established over time, it isrecommended that sampling time be extended into the future.This could be achieved, for instance, by verification of quality during other seasons of theyear, e.g. during wet weather flow, as during such times less diversion of grey water wouldbe expected.

[发布机构] CSIRO

[学科分类] 地球科学(综合)