COMPARISON OF DECENTRALISED AND CENTRALISED WASTEWATER SERVICING APPROACHES: PARK ORCHARDS CASE STUDY

COMPARISON OF DECENTRALISED AND CENTRALISED WASTEWATER SERVICING APPROACHES: PARK ORCHARDS CASE STUDY Ben Asquith 1, Joshua Eggleton 1, Glenn Wilson 2 1 BMT WBM, Newcastle, NSW, ben.asquith@bmtwbm.com.au 2 Yarra Valley Water, Melbourne, Vic ABSTRACT The evaluation of the relative benefits, costs and risks between centralised and decentralised wastewater servicing strategies can be challenging for water utilities. This paper presents one cost effective approach that has been tested by the authors on behalf of Yarra Valley Water through a case study for Park Orchards. The project involved development, testing and life cycle cost analysis of alternative long-term wastewater servicing strategies to conventional reticulated sewerage that involved the retention, upgrade and in some cases replacement of existing on-site systems. The case study identified that implementation of a series of practicable upgrades to existing on-site systems had the potential to achieve significant improvement in health and ecosystem protection. This has significant implications for the Victoria Sewerage Backlog Program given the rising costs to deliver reticulated sewerage to these areas. INTRODUCTION YVW are considering alternative approaches to provide a sustainable wastewater management service other than conventional reticulated sewerage. Ideally, these alternatives would deliver an equivalent level of ecosystem and health protection as reticulated sewerage. However, as a minimum YVW are considering the potential for lower cost solutions to deliver a satisfactory improvement in performance based on the legislative objectives set out in the State Environmental Protection Policy (SEPP) Waters of Victoria. Park Orchards was selected by YVW as a case study to evaluate the potential for alternative servicing scenarios based on community concern over the provision of reticulated sewerage and estimated capital costs. The project aimed to evaluate the potential to retain some level of on-site management of sewage from Sewerage Backlog Areas, using the Park Orchards Backlog area (RA39) as a case study. The project involved field and desktop evaluation of natural and built conditions within the Study Area and construction of numerical models to estimate the capability of existing on-site systems to meet ecosystem and health protection objectives. Following characterisation of existing conditions, the maximum receiving capacity (via land application) of lots within the Study Area was estimated through comprehensive spatial analysis and modelling. The outcomes of these tasks allowed a number of potential wastewater servicing scenarios to be examined to test the potential for long-term containment of wastewater on-site. These scenarios involved the hypothetical upgrade of existing on-site systems where on-site containment was considered possible. Sites not considered capable of full on-site containment were assessed under partial containment and connection to sewerage scenarios. The outcomes of this analysis were used to classify each lot in the Study Area based on the potential for full, partial or no on-site containment of wastewater. They also allowed consideration of the relative benefits of different upgrade scenarios in comparison to the existing case and full connection to sewerage. Finally consideration was given to the relative cumulative impacts of on-site systems both between servicing scenarios and compared to background (i.e. non-wastewater) loads. PARK ORCHARDS STUDY AREA The Park Orchards Backlog area contains approximately 1,25 existing on-site wastewater management system. The majority of Park Orchards is located within the Study Area along with a small part of Ringwood North and Warrandyte South. It is located on the north eastern fringes of Melbourne approximately 4 kilometres south of the Yarra River. The Park Orchards Study Area is located within the rolling hills of the Yarra hinterland at an elevation range of 6 15 metres AHD with local relief of 1-5 metres typical. Slopes on crests and ridges are typically 15% whilst mid to lower slopes are 1 5% creating significant constraints to the land application of effluent. Park Orchards has a temperate climate with warm summers and cool winters. Mean temperatures range between 6 and 26 degrees. Average annual rainfall and pan evaporation for the site are 832 and 125 mm respectively. The site experiences

moderate rainfall in Autumn and Spring, compounded by a significant drop in plant water demand. The climate does place limitations on the capacity for vegetation to utilise effluent from onsite systems, particularly from April to September. Soils are typical of those formed on uplifted Silurian sedimentary geology. Total soil depth is highly variable with the profile consisting of weak to moderately structured loams grading to clays to a depth of.5 1 metre common. Subsoils displayed some potential for sodicity. Phosphorus sorption capacity was moderate to very high. The Park Orchards Study Area contains a network of incised ephemeral watercourses (largely a result of the erosion of the sedimentary parent material) that eventually drain to the Yarra River via Andersons Creek and Mullum Mullum Creek. Beyond the study area both Mullum Mullum and Andersons Creek meander northward through the low hills and alluvial plain to the Yarra River. Existing water quality (summarised in Table 4) data was limited but indicated that nutrient and pathogenic indicator concentrations are elevated in comparison with Low Risk Trigger values and EPA Victoria Environmental Quality Objectives. METHODOLOGY This Study involved the use of GIS analysis and environmental modelling to evaluate the capacity for on-site containment of wastewater. BMT WBM has built a spatially and temporally varying model of the Park Orchards Study Area that takes into account; climate (rainfall, temperature, evapotranspiration); topography (slope, elevation); soil characteristics (depth, hydraulic and chemical properties); vegetation (transpiration, nutrient uptake); hydrology (rainfall-runoff, soil water balance); background pollutant loads (stormwater quality); land use (imperviousness, background pollutant loads); wastewater generation (water use data); on-site system characteristics (for a limited number of typical system types); and catchment attenuation of pollutants (accounting to proximity to receiving waters). This model has the ability to continuously simulate the key processes governing the performance of on-site systems over extended periods (i.e. decades). As a result, information on containment of wastewater on-site and the proportional contribution of on-site systems to nutrient and pathogen loads was able to be estimated. Once the existing (or base) case was characterised, the model was used to test a number of hypothetical wastewater management scenarios to determine the relative change in on-site containment potential and off-site impacts. Information / Data Analysis A Geographical Information System (GIS) workspace was established that allowed the range of available data to be interrogated and used to broadly characterise the subject site. It also assisted in targeting the field investigation program towards critical information gaps. The key data gaps with the potential to influence study outcomes are as follows. It was originally thought that LiDAR data would be available to create a highly accurate Digital Elevation Model (DEM). However, this proved unfeasible. As an alternative, a DEM was created from the 1 metre contour data and then refined to account for the influence of the reticulated stormwater system and roads. Published soil landscape mapping was not made available for this study. This did not prove to be a significant impediment to the accuracy of study outcomes. Local soil variation (not identifiable in broad scale mapping) was significant and little would be gained from the availability of landscape mapping. Similarly, field soil investigations involved a limited number of observation sites to assist with development of soil parameters. There will invariably be site to site variation in soil characteristics that have not been accounted for in this assessment. Limited site specific data on the nature and extent of existing on-site systems was available. A range of assumptions were made to enable the systems to be modelled. No suitable streamflow, water quality or wastewater discharge data was available that was suitable for full parameterisation / calibration of the models. Land Capability Assessment and Mapping BMT WBM has completed a Land Capability Assessment (LCA) for the Park Orchards Study Area. The purpose of the LCA was to collect a range of biophysical information for assessment of the potential for on site wastewater management. Site and soil characteristics were then used to identify and rate conditions with the potential to limit the performance of on site wastewater systems. The approach was comparable to the LCA procedures set out in the EPA Victoria Land Capability Assessment for On-site Domestic Wastewater Management, Publication 746.1 (23). However given that this study involved consideration of land capability based on limited field data and broad scale spatial information, the approach was modified to account for the inherent

uncertainty associated with a broad scale (non site specific) assessment. The process, structure and rating system adopted for this LCA follows a slightly altered framework compared to that outlined in EPA Publication 746.1 (23). Lots have been classified as Low, Medium, High or Very High Hazard. Lot Area Available for Effluent Management An evaluation of allotment area available for effluent management was required in order to estimate the capacity for on-site containment of wastewater. A statistical analysis has been undertaken of a representative sample of allotments from within Park Orchards to identify; the relationship between allotment size and proportion of lot available for effluent management; and the relationship between allotment land capability and the land capability of available area. Raw available area was considered to be the land remaining on each sample allotment, after subtracting the area occupied by development and required separation distances. An assessment was undertaken of a representative sample of allotments within the Study Area. A total of 139 allotments were assessed to determine the capacity to provide available area for on-site wastewater management. The assessment was undertaken through orthophoto investigations and GIS creation of buffers around the abovementioned objects. Statistics on the area of land and proportion of total lot area occupied by each component (inclusive of buffers) were recorded for analysis. Statistics obtained from this assessment were analysed to identify any patterns or relationships between lot size and area available for effluent Land Application Areas (LAA s). A scatter plot of lot size and the proportion of the lot unavailable for effluent management was created to determine an overall relationship that could be applied to the entire Study Area. Further statistical analysis was undertaken to determine the proportion of raw available area for each sample lot that was identified as Very High Hazard (VHH) through the Land Capability Mapping. For the 139 sample lots the ratio of VHH area to lot size was compared against the ratio of VHH area to the Raw Available Area polygon for that lot. The identified relationship was consequently applied to all study lots to calculate the VHH area present within the Raw Available Area. This VHH area was then excluded from the Raw Available Area to calculate the Final Available Area for each allotment in the Study Area. Background Pollutant Loads Stormwater quantity and quality modelling was completed in order to evaluate the relative contribution of on-site systems within the Study Area to catchment nutrient export. The Model for Urban Stormwater Improvement Conceptualisation (MUSIC) was applied to estimate continuous hydrology and runoff water quality for the catchment. A MUSIC model was prepared for the existing catchment scenario. Stormwater quality modelling was undertaken to develop an appreciation of the catchment water balance and estimate stormwater pollutant loads for representative parameters. Estimation of runoff volumes and loads of common stormwater pollutants including Total Suspended Solids (TSS), Total Phosphorus (TP) and Total Nitrogen (TN) was completed. The MUSIC modelling approach applied to assist in estimating existing background pollutant loads for the catchment is described in the following sections. Wastewater Management System Performance Water, nutrient and pathogen modelling has been undertaken using Decentralised Sewage Model (DSM). The Decentralised Sewage Model (DSM) is a GIS based decision support tool designed to assess and compare a range of wastewater servicing options from on-site sewage management to conventional gravity sewerage with central treatment and reuse/disposal. The DSM was developed jointly by BMT WBM and Whitehead & Associates Environmental Consultants. It has the capacity to rapidly assess the long-term environmental/human health performance of wastewater systems in addition to assisting in the concept design and costing of various servicing options. Adoption of a daily continuous water, nutrient and pathogen modelling approach is considered a superior approach to calculation of minimum land requirements using a lumped monthly water balance. Monthly water balances are a very conservative (but simple and quick) tool for sizing new effluent land application areas. They operate on the principle that a significant factor of safety is being built into the design to account for unforseen operational issues or short-term peak loading. When coupled with a design wastewater flow based on 5-6 persons (9 1,8 litres/day), the lumped monthly approach produces minimum land area sizes that are typically 2 4 times the required size under average conditions. Such an approach is neither realistic nor necessary when considering the potential for on-site containment in an existing area such as the Park Orchards Study Area.

A total of five wastewater servicing scenarios were evaluated using the DSM; the existing case; Scenario 1A; Upgrade Existing Systems to Best Practicable Option (No Reticulated Sewerage); Scenario 1B; Upgrade Existing Systems to Best Practicable Option (with Reticulated Sewerage for lots with limited to no potential for onsite containment); Scenario 2A; Replace Existing Systems with Best Practice Option (No Reticulated Sewerage); and Scenario 2B; Replace Existing Systems with Best Practice Option (with Reticulated Sewerage for lots with limited to no potential for onsite containment). To allow construction of a DSM model for each of the alternative servicing scenarios, a logical set of upgrade rules were developed based on the experience of BMT WBM, liaison with YVW and MCC and the outcomes of field investigations. These rules considered the following factors in assigning an upgrade or replacement on-site system option to a site. A broad upgrade / replacement logic. Typical site and soil conditions, existing development conditions (house layouts etc) and other constraints to on-site system siting and construction. Logical and cost effective approaches to improve the performance of the existing systems (i.e. value for money). Avoiding alteration to systems likely to already be achieving full or high proportions of on-site containment. Minimising direct off-site discharge wherever possible. The key factor determining the nature of adopted upgrade / replacement options was the potential for on-site containment of wastewater. For the purposes of DSM modelling on-site containment was defined as follows: An on-site wastewater management system can be considered to be achieving full on-site containment where off-site discharge (overflow) or hydraulic failure (surcharge) of land application systems is calculated to be zero for the entire thirty year DSM simulation. Average annual flows and loads from DSM scenarios and MUSIC (background) loads were combined to generate an overall mass balance model for the Study Area for each scenario. Based on the average annual flows and loads the average annual pollutant concentrations exported from the Study Area under each scenario can also be calculated. Study Area Mass Balance and Risk Maps Average annual flows and loads from DSM scenarios and MUSIC (background) loads were combined to generate an overall mass balance model for the Study Area for each scenario. Based on the average annual flows and loads the average annual pollutant concentrations exported from the Study Area under each scenario can also be calculated. The frequency of overflow / surcharge is often used as a measure of performance for on-site systems. Based on the frequency of overflow / surcharge a series of on-site containment maps were produced for each scenario. The criteria for the on site containment maps is presented in Table 1. Table 1 Criteria for On-Site Maps Overflow / Surcharge (>) Frequency (% of years) >-5 5-<1 1 Potential for On-Site Potential for Full Potential for Partial to Full Limited Potential for Partial No Potential for RESULTS AND DISCUSSION Land Capability Assessment A summary of the breakdown of land capability for the Study Area is provided in Table 2 and Figure 1. It can be seen that the majority of the Study Area was classified as High to Very High Hazard with respect to land capability. The two dominant constraints across the Study Area were slope and the shallow, sodic Upper Slope soils. These constraints were compounded where a site was in close proximity to a watercourse or environmentally sensitive zone (ESO) such as the 1 Acres. Table 2 Land Capability for Park Orchards Hazard Class Number Land Area (ha) Low Hazard 1 17 3% Medium Hazard 2 25 4% High Hazard 3 197 32% Very High Hazard 4 374 61% Percentage

Background Pollutant Loads Annual stormwater flow and pollutant load estimates were obtained from the MUSIC modelling for existing development conditions. The results are summarised in Table 4 with modelled concentrations compared to recent mean water quality for Mullum Mullum and Andersons Creek. Table 4 Existing Scenario Modelling Results Load (ML or kg/yr) Flow 3,63 Mean Annual Concentration (mg/l) Mullum Mullum Creek (mg/l) Andersons Creek (mg/l) TP 917.25.18.17 TN 7,36 2 1.75 2.9 Figure 1: Final Land Capability Map Lot Area Available for Effluent Management This assessment used allotment size and land capability as the determining factors in estimating available area. The results of the Final Available Area analysis (i.e. including consideration of land capability) are presented in Table 3. Table 3 Final Available Area Statistics for Park Orchards Range Number of Lots Percentage <1 m2 141 11% 1 3 m2 611 49% 3 6 m2 293 24% >6 m2 196 16% It is important to recognise the limitations of the available area assessment conducted as part of this Study. Results should only be considered an approximation of available area given the relatively poor correlation between allotment size and available area. Limited field and desktop groundtruthing of available area has been undertaken by BMT WBM. The groundtruthing found the predicted available area to be approximately correct in the majority of cases. There were however, allotments identified where Final Available Area was underestimated or overestimated. Overestimation of available area was less common, making the results relatively conservative. The result compare favourably to measured longterm water quality when consideration is given to broader land use and pollutant attenuation during streamflow. Mass Balance Modelling The results of DSM and Study Area mass balance modelling are summarised in Table 5 Table 6 and Figure 2 Figure 6. The focus of results is on longterm average conditions as they are most representative of long-term performance. Temporal variation in on-site system and background pollutant loads was evaluated and found to be limited. Table 5 summarises the key statistics relating to on-site containment for each scenario. It can be seen that even under existing conditions, approximately two thirds of allotments / exiting onsite systems are likely to meet the containment criteria or have potential to meet the criteria. However, this proportion increases to 96% under Scenario 1A (best practicable upgrades no reticulated sewerage). Results for Scenario 1B (where on-site systems were only retained where full containment under typical loading was achieved) suggest that sewerage connection would be required for approximately 27% of allotments. Scenario 2A (best practice upgrade no reticulated sewerage) produced a poorer performance in terms of on-site containment than 1A. This relates to the conservative nature of a soil moisture monitoring system being used to schedule effluent application where for sites where containment was not possible effluent (albeit highly treated) was directed to stormwater more often. Once these systems are removed (under Scenario 2B where approximately 21% of allotments are connected to sewer) a very high level of service is achieved based on results of the modelling.

Table 5 Summary Statistics for On-site Capacity Classification Existing 1A 1B 2A 2B Full Partial / Full Partial / No 44% 59% 75% 74% 93% 21% 37% 25% 5% 7% 1% 1% 12% No 34% 3% 9% Note 1 Percentage of existing systems retaining some level of on-site effluent application under each scenario. Figure 3: Average Annual Phosphorus Export Table 6 Indicative Upgrade Program Scenario 1A Scenario 1B Scenario 2A No Change 423 (34%) 135 (11%) Retain ST/SSI 47 (38%) 456 (37%) Retain PT/LPED 64 (5%) AST/SSI 382 (31%) AST/LPED/OSD Retain ST/LPED/OSD 149 (12%) 123 (1%) Scenario 2B AST/SSI/OSD 81 (7%) Retain ST/SSI/OSD 171 (14%) AST/OSD 12 (1%) 16 (1%) Connect to Sewer 284 (23%) 268 (22%) PT: Primary Treatment, ST: Secondary Treatment, AST: Advanced Secondary Treatment, SSI: Subsurface Irrigation, LPED: Low Pressure Effluent Dosing, OSD: Off-site Discharge. Figure 4: Average Annual Virus Concentration Figure 2: Average Annual Nitrogen Export Figure 5: On-Site Map Existing

Figure 6: On-Site Map Scenario 1A OUTCOMES A number of useful outcomes have been obtained from this Study. An improved understanding of the constraints to sustainable on-site wastewater management has been gained through land capability mapping and an analysis of available area for effluent land application. The Park Orchards Study Area is highly constrained with respect to land capability with key issues being slope, shallow soil and sodic subsoils in the upper to mid slopes. This is further compounded by the small to medium allotment size and substantial existing development observed on some sites (tennis courts, swimming pools, large dwellings). Notwithstanding, the majority of existing allotments are likely to contain some suitable land for effluent land application (2 4 m2 typical). However, land capability restrictions along with the documented limitations to the available area analysis mean a more detailed Land Capability Assessment (LCA) should be completed for any site where long-term on-site sewage management is planned to be retained as an alternative servicing scenario. Existing on-site systems are a moderate to significant contributor to catchment nutrient loads and present a significant risk to human health. Systems that involve partial or full off-site discharge account for the majority of this impact. Removal of as many off-site discharges as possible is the single most effective management strategy for wastewater impacts. Opportunities to maximise the land application of effluent (at sustainable loading rates) should be identified on a lot by lot basis. Alternative Wastewater Servicing Scenario 1A (Best Practicable Option) has been identified as a highly effective approach to managing wastewater impacts. The approach does carry some potential risk with respect to the constructability of LPED trenches (slopes and shallow soil). Alternative Wastewater Servicing Scenario 2A (Best Practice Upgrades) is likely to offer limited benefit for almost double the capital cost. However, a higher number of best practice upgrades may be necessary should the construction of LPED systems prove not to be feasible for some sites. Alternative Wastewater Servicing Scenarios 1B and 2B (combined on-site and sewerage solutions) are not identified as cost effective management options. Limited additional benefit is provided with respect to ecosystem and health impacts. A choice between full reticulated sewer and total on-site wastewater management (with some on-going offsite discharge where necessary) is likely to be preferable. Existing on-site systems are a minor contributor to nitrogen and moderate contributor to phosphorus loads exported from the Study Area. A reduction in non-wastewater nutrient loads would be required to reduce average annual concentrations to a level comparable with EPA EQO s. There is a limit to the effectiveness of investment in improving on-site systems or in fact provision of sewerage services from a nutrient management perspective. A number of potential on-site wastewater management solutions have been considered as part of this Study. Concept drawings and descriptions are provided for further consideration of feasibility. Implementation of an Alternative Wastewater Servicing Scenario (Something between 1A and 2A) is likely to cost in the order of $6,5 $14,7 per lot. The framework for design, construction, ownership and operation of on-site systems under an alternative servicing scenario requires careful consideration. The outcomes of this Study are based on an assumption of centralised management and ownership. The definition and interpretation of on-site containment is complex and requires liaison with EPA Victoria. Requiring zero hydraulic failure of land application systems over their design life would be grossly conservative and inequitable with other regulatory targets for the frequency of pollutant or stormwater discharge to waterways. ACKNOWLEDGEMENTS BMT WBM wishes to thank Yarra Valley Water for allowing this case study to be published. We also thank Glenn Marriott from Landsafe Ag-Challenge for assistance with this project. REFERENCES EPA Victoria (23) Land Capability Assessment for On-site Domestic Wastewater Management, Publication 746.1.

Table 7 Summary of Indicative Cost Estimates for Alternative Servicing Scenarios Scenario 1A Scenario 2A Reticulated Sewer Low High Low High Capital $8.1M $1.6M $13.9M $18.2M $32.4M Capital (per lot) $6,5 $8,57 $11,176 $14,691 $26,143 Operational (p.a.) $.7M $.7M $.814M $.814M $.21M 25yr NPV -$17.6M -$21.3M -$23.5M -$28.5M -$32.8M NPV (per lot) -$14,164 -$17,152 -$18,922 -$22,965 -$26,39 Costs as % of (NPV) Reticulated Sewer 54% 65% 72% 87% 1%