Optimining Wastewater Treatment & Disposal Capital Costs Through Targeted Infiltration and Flow Reduction

Initial investigations for future wastewater treatment schemes in Featherston showed that infiltration and inflow (I/I), especially groundwater infiltration (GWI), was at such a level that the cost of any scheme would be highly influenced by the excessive flow volume. This project was commissioned to determine the level and extent of network rehabilitation required to achieve the lowest overall cost for future high rate treatment and land based treatment and disposal options.

The first phase of investigations was to determine the extent of GWI and the cost of addressing it. A night flow study showed that GWI was relatively isolated with 83% of the measured night flow coming from 23% of the total network pipe length (top 5 of 17 monitored sub-catchments). Flow reductions and associated rehabilitation costs were estimated for each sub-catchment using rehabilitation effectiveness estimations and current rehabilitation costs. The diminishing return on investment was clearly evident beyond the top 5 sub-catchments. 

The capital costs of high rate treatment and land based treatment and disposal schemes were calculated for different levels of flow reduction achieved through I/I rehabilitation. This allowed the optimum financial solution for the complete upgrade (including I/I rehabilitation) to be found, balancing the savings on treatment and disposal against the increasing rehabilitation costs. With no I/I reduction the capital cost of a high rate treatment plant and a land based treatment and disposal scheme was estimated at $15.4M and $18.8M, respectively. The most economical high rate treatment scenario was achieved through $0.98M of network rehabilitation works which would result in a 27.5% reduction in ADWF and achieve a $2.6M capital cost saving compared with no I/I reduction. The total project net saving under this scenario was estimated at $1.62M including the cost of network rehabilitation. The most economical land disposal scenario was reached at $1.48M of network rehabilitation, resulting in a 31.4% reduction in ADWF which would achieve a $5.71M capital cost saving compared with no I/I reduction. The total project net saving under this scenario was $4.23M including the cost of network rehabilitation.