BENCHMARKING ENERGY USE FOR WASTEWATER TREATMENT PLANTS
A summary of the 2015-16 benchmarking study
D de Haas, G Appleby, G Charakos, N Dinesh
Publication Date (Web): 4 May 2018
Understanding energy use of a WWTP plant, and benchmarking it against other similar plants, is an important step to reducing energy-related costs and greenhouse gas emissions. In 2013-14, the Water Services Association of Australia (WSAA) conducted a first-round of energy benchmarking for 142 WWTPs across Australia. A second-round benchmarking exercise was performed by WSAA and the Intelligent Water Network (IWN) for the 2015-16 data period, which included 243 WWTPs in Australia and two WWTPs in Auckland, New Zealand. It represents a 73% increase in WWTP participation from the previous round.
This paper summarises the findings of the second benchmarking round by WSAA-IWN and compares the results to those of the previous round. In both rounds, the primary benchmark applied was specific energy use (kWh) per unit of equivalent population (EP) per year (based on raw influent loads). A secondary benchmark of energy self-supply was applied for plants with on-site renewable energy, predominantly co-generation from biogas. Alternative benchmarks of specific energy use per unit of pollutant removed were also investigated.
The benchmarking results represent the consolidation of outputs from a large database of discrete and time-series data submitted by 31 water utilities that participated in the 2015-16 study. Of these, around half (17 utilities representing 121 WWTPs) had also participated in the previous benchmarking round (in 2013-14). Within that sub-set, slightly more than half the WWTPs showed some improvement in energy efficiency, and slightly over one-third showed an improvement of more than 10%. The secondary benchmark of self-supply of energy improved by 7% on average for applicable WWTPs, compared to the first round of benchmarking. These are significant improvements and reflect the efforts of water utilities to improve energy efficiency. It also demonstrates the value of benchmarking as a tool in continuous improvement. For example, most of the WWTPs added in the latest round were extended aeration-type activated sludge plants and aerated lagoons. Typically, these types tend to have relatively poor energy efficiency. The results can help to identify and prioritise plants that have significant opportunities for improvement, relative to Target and Guide Values for their peers, based on similarities in benchmark Type and Size Class.
The study also examined alternative benchmark metrics that express energy use per unit mass of pollutant (organic material or nitrogen) removed. It found that metrics based on removal of organics (kWh/kg removed as chemical or biochemical oxygen demand) offered little or no advantage over the primary benchmark adopted here of kWh/(EP.y), where equivalent population is derived from raw wastewater loading. Reasonably reliable conversion between these metrics is feasible since the removal of COD or BOD is typically nearly complete (90% or greater, on average) for most plants, with the possible exception of lagoon-type plants that have lower removals (around 80% on average). However, nitrogen (N) removal varies more widely (ranging approximately 20% to 98% in this study), being more heavily dependent on WWTP type, configuration and loading. Hence, when comparing different plants, applying alternative metrics for specific energy use that include kWh/kgN removed might be more useful.
A number of areas for future improvement can be identified from the study. These include: data collection; data handling and sharing for benchmarking purposes, particularly in the context of ‘big data’; data validation and security around shared information; and future legacy. For example, there is a need for continued cooperative efforts to agree on additional and/or future benchmarks, both in terms of metrics and Target and Guide Values, within the Australian/New Zealand wastewater treatment context. This study also highlighted the need particularly to better systematise the distinction between wastewater pumping vs. treatment, which presents challenges to the way in which energy and related parameters (e.g. flow, concentration, pressure) are measured or sub-metered, and reported for benchmarking purposes. Furthermore, more than 120 WWTPs were considered ‘below average’ and not energy efficient. For these plants, there remains significant opportunity to improve energy efficiency and/or lower energy use.
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