Small scale combined wastewater polishing and biomass production: a case study
The overarching aim of this project was to establish a site to demonstrate the application of combined Willow short rotation coppice constructed wetland technology at a small scale within an existing rural wastewater treatment system.
Low energy, low maintenance solutions are required for the polishing of effluent from small wastewater treatment plants (WWTPs), in particular to meet increasingly stringent standards for N and P discharge to receiving waters (Dong et al. 2013). It is possible to combine wastewater treatment (WWT) and production of biomass for energy generation and this has been widely demonstrated through irrigation of willow plantations with wastewater (with or without subsequent recapture). This has proven performance at operational scale in Europe (Börjesson & Berndes 2006) – for example, Willow plantations are irrigated by wastewater from the town of Enkoping, Sweden, and the biomass produced is used in the local district heating system (Mola-Yudego & Pelkonen 2011). In this case, 1000 ha of short rotation coppice SRC provides 15% of the wood-fuel for a combined heat and power (CHP) plant which supplies 20,000 inhabitants and removes N, P and other pollutants that otherwise would enter Lake Mälaren. Ash from the boiler is recycled back to the plantation; hence cost savings in waste disposal, fuel supply and environmental protection occur at various points in the waste-to-energy cycle. The scheme involves close cooperation between the municipality, the Energy Company and local farmers. SRC has been identified as one of the most energy efficient carbon conversion technologies to reduce greenhouse gas (GHG) emissions (Don et al. 2012). The Water Renew LIFE (2004-08) project (Waterwise, 2011) demonstrated that the concept has significant promise for the UK. However, concerns remain about long term impacts on groundwater quality (Nissim et al. 2015) and the approach may not be suitable where river baseflows depend on inputs from treated wastewater.
In the UK, there is wide use of constructed wetlands (primarily reed beds) for final polishing of effluent from WWTPs. Pilot studies have shown that Willow can be used in a lined constructed wetland configuration (Bialowiec et al. 2012), yet there is a paucity of research or practical application of this practice for domestic wastewater treatment in the UK. We propose that Willow Short Rotation Coppice (SRC) can be applied by establishing constructed wetland-type designs where Willow replaces conventional macrophytes, utilising existing infrastructure and by-passing regulatory concerns relating to potential groundwater contamination or diminished river base flows. Constructed wetlands planted with Willow SRC rather than conventional macrophytes have the potential to provide the same wastewater polishing function with the added benefit of producing biomass. Current targets for renewable energy and GHG reduction make this especially timely.
Several types of CW exist, the most common in being horizontal sub-surface flow designs which typically comprise a lined bed containing a physical medium, usually gravel. There is usually a continual flow of wastewater from where it is deposited on the surface of the bed at the inlet end, through the bed a few centimetres below the media surface to a collection pipe at the effluent end (usually height-adjustable to maintain an appropriate water level as the bed ages and the hydraulic regime changes with deposition of fines and rhizosphere development). Constructed wetlands are typically planted with one (or more) wet-adapted plant species (aquatic macrophytes). The most commonly used plant species in such systems is the common reed Phragmites australis. This species thrives in wet conditions, tolerating high nutrient loadings. Vegetation is usually not harvested from wastewater treatment wetlands (Vymazal 2002) as harvesting for the purpose of nutrient removal is generally considered impractical for the amount of nitrogen and phosphorus removed (Kadlec & Wallace 2008). Harvested reed biomass tends to present a problem in terms of disposal (haulage and composting or digestion costs). However, biomass from SRC could be harvested and utilized directly and locally for energy generation or other local needs, or for off-setting energy costs within the wastewater treatment industry.
Willow SRC is better adapted to a flow-regime of frequent wetting and draining cycles rather than standard horizontal sub-surface flow wetland conditions. Consequently, vertical or tidal flow CW configurations are likely to be more appropriate hydraulic regimes for this application. Tidal flow configuration can increase the average oxygen supply substantially compared with conventional constructed wetlands resulting in enhanced removal of BOD and NH4+ (Bialowiec et al. 2011, Wu et al. 2011). Tidal flow configuration was established for the constructed wetlands at Dinnet.
- Low energy, low maintenance solutions are required for the polishing of effluent from small wastewater treatment plants (WWTPs), in particular to meet increasingly stringent standards for N and P discharge to receiving waters.
- It is possible to combine wastewater treatment (WWT) and production of biomass for energy generation and this has been widely demonstrated through irrigation of willow plantations with wastewater.
- In April 2013, a demonstration constructed wetland site was established at Dinnet, Aberdeenshire, which combined short coppice willow and constructed wetland technology within an existing rural wastewater treatment system. A more conventional reed bed was installed alongside the willow bed.
- A tidal flow configuration was installed to increase the oxygen supply in the treatment beds compared with conventional constructed wetlands to enhance removal of BOD and NH4-N.
- A full sampling regime to test effects on water quality, biomass production, microbial and invertebrate diversity and greenhouse gases was implemented in 2014-5. Additionally, substrate efficiency was tested in a separate controlled replicated experiment.
- The reeds fairled to establish well, however the bed performed within expected efficiency ranges for most parameters. Removal rates for concentrations of BOD, NTot, NH4-N and PTot were 74, 36, 38 and 52 % respectively.
- The willow bed performed more efficiently than the reed for a number of water quality parameters including reducing concentrations of NTot (51 %), NH4-N (53 %), PTot (64 %). However, greenhouse gas emissions were higher from the willow treatment bed.
- In terms of establishment, growth and biomass production, willow appears to be a more suitable plant choice for this particular area and climate.
- Both treatment beds had a neutral effect on biodiversity compared to the original soak away area.
- This low energy, low cost and low maintenance technology holds great promise for treating municipal wastewater in rural communities in Scotland.
Staff working on this project
- Lisa Avery
- Luke Beesley
- Jenni Stockan
- Mads Troldberg
- Rupert Hough
- Matteo Tamburini
- Peter Randerson
