Abstract
The application of water treatment residuals (WTRs) has been proposed as a mitigation option for reducing the risk of phosphorus (P) loss from high P impacted soils. High P impacted soils can arise from repeated application of biosolids and other P fertilisers over time. WTRs which are rich in alum and consequently may sequester phosphorus, may be mixed with P sources and P impacted soils, thereby reducing any environmental risk. To investigate this issue, a research programme was conducted to (I) investigate the role of influent water type on WTR physio-chemical properties and P adsorption capacity, (2) quantify the impact of co-mixing WTR and biosolids on surface runoff and leaching of contaminants, and (3) measure the impact of WTR/biosolid additions on short rotation coppice (SRC) willow growth and nutrient uptake. Investigating the application of WTR to SRC willow specifically is justified on the grounds that SRC willow is a bioenergy technology which offers the potential for low cost safe disposal of biosolids and WTR, without any risk to contamination of the human food chain.The findings of batch studies indicate that WTRs from various sources of influent water type varied significantly in P adsorption capacity. Factors influencing variation include background A I3' concentration, which confounds the relationship between P sorption and oxalate extractable A! -+ Fe used to determine P indices. Furthermore, a negative correlation was also observed between P sorption and total carbon (TC). Further research is required to explore variation over time and identify specifically the nature of carbon which influences P adsorption, as well as refine the method used to determine P adsorption.
Two rainfall simulation studies investigating the effects of (1) varying rates of combined WTR and biosolid application, and (2) varying the application methods of WTR on soils applied with a fixed rate of biosolid , found that losses of P via overland flow (OLF) and leachate were unaffected by WTR incorporated within the mixed soil and biosolid profile. Results indicate that negligible adsorption of soluble P by WTR occurs during the first 16 days when WTR is incorporated into soil. However, WTR does have a significant effect on soil surface hydrological processes. When WTR was surface applied at a rate of 65Mg ha'1 on soil mixed with 52Mg ha'1 biosolid, no OFF was generated over two rainfall events, 13 days post WTR application. Results suggest that properties within WTR namely, A1 and Fe oxides, and organic matter resisted the impact of rainfall on aggregate breakdown. This result contrasts with the incorporation of WTR with biosolid in the soil, where the effect on incidental P loss is insignificant due to overriding hydrological factors.
WTRs had no negative effects on the willow genotypes tested during three- month pot trials either applied alone or with biosolid. Plant P concentration was reduced by WTR but this had no negative effect on biomass yields. Moreover, soil test P was significantly reduced with increased WTR application. Willow biomass and plant P concentrations were similar for controls and treatments at the highest rate of WTR and biosolid adopted. Thus, WTR improved the soil P storage capacity of the soil with no reduction in plant growth.
On the basis of these hidings, WTR could be used to mitigate incidental P loss via overland flow in SRC willow plantations recycling biosolids. WTR could also be incorporated in soils to reduced long term soil test P and residual P transfers, provided subsurface hydrological risks and long term effects on plant performance are quantified. However, WTR properties need to be considered when forming indices for potential P adsorption capacities and future research should investigate WTR properties which provide resistance to aggregate breakdown under various hydrological conditions.
| Date of Award | Jul 2019 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Sponsors | Department of Agriculture, Environment and Rural Affairs |
| Supervisor | Raymond Flynn (Supervisor), Alistair McCracken (Supervisor) & Donnacha Doody (Supervisor) |
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