Recovery of Nutrients from Biogas Digestate with Biochar and Clinoptilolite

Research output: Book/ReportPh.D. thesisResearch

  • Nazli Pelin Kocatürk
The increasing number of biogas plants over the last decades has brought the need to improve techniques to handle digestate, the by-product of anaerobic digestion in biogas plants. Separation of digestate into liquid and solid fractions is often applied in centralised biogas plants, which necessitates the subsequent distribution of nutrients. The liquid fraction of digestate can be used as fertiliser in agricultural crop production systems and the most common practice of utilising the liquid fraction of digestate is direct field application in the vicinity of the biogas plant. However, direct application may result in practical problems such as need for high storage volume, and environmental problems as a result of nutrient losses in the environment. To overcome such problems, recovery and concentration of nutrients from the liquid fraction may be a desirable option which, would also result in recovery of nutrients whose natural reserves are being depleted such as phosphorus and potassium. In this thesis I propose the use of sorbents i.e. biochar and clinoptilolite to concentrate nutrients and subsequently the application of digestate-enriched biochar and clinoptilolite as fertiliser. Therefore the overall objective of this thesis is to investigate the use of clinoptilolite and biochar to recover plant nutrients from the liquid fraction of digestate resulting from anaerobic digestion of animal manure and investigate the plant-availability of the recovered form of nutrients.
In Chapter 1 (General Introduction), I summarised the motivation for this thesis, the objectives and experimental approach.
In Chapter 2, I investigated the use of clinoptilolite as a sorbent to recover nutrients from the liquid fraction of digestate. I found no effect of preconditioning on the total ammonium and potassium removal from the liquid fraction of digestate. I also found that the removal efficiency of ammonium, potassium and orthophosphate from the liquid fraction of digestate was significantly affected by the initial loading ratio; and increasing initial loading ratios resulted in increasing removals (concentration of nutrients on sorbent) but decreasing efficiencies of clinoptilolite to remove nutrients from the liquid fraction of digestate.
In Chapter 3, I studied the chemical activation of biochar by treating the biochar with deionised water, hydrogen peroxide, sulfuric acid and sodium hydroxide solutions to investigate the effects of activation on oxygen-containing functional groups, and ammonium, orthophosphate and potassium removal from the liquid fraction of digestate. I found that ammonium, potassium and orthophosphate removal from liquid fraction of digestate was improved by chemical activation. FTIR photoacoustic spectroscopy revealed that orthophosphate removal was related to oxygen-containing functional groups. However, no strong correlation was found between the oxygen-containing functional groups and ammonium removal from liquid fraction of digestate.
In Chapter 4, I investigated the effects of initial loading ratio on the ammonium, potassium, orthophosphate removal efficiency by biochar from the liquid fraction of digestate. I observed similar effects of initial loading ratio on the biochar (holm oak wood pyrolysed at 650 °C) as for clinoptilolite. I also investigated the effect of the combination of clinoptilolite and biochar on the nutrient removal efficiency and found that combining biochar with clinoptilolite did not improve the efficiency of clinoptilolite to remove ammonium, orthophosphate, total N and dissolved organic carbon except for total P; and decreased potassium removal efficiency compared to clinoptilolite alone.
In Chapter 5, I tested nutrient enriched biochar and clinoptilolite as nitrogen fertilisers with a pot experiment using the double-pot technique with ryegrass. I found that N sorbed to clinoptilolite and biochar from liquid fraction of digestate was available to plans and both enriched biochar and clinoptilolite were able to increase biomass yield and N uptake. However, clinoptilolite resulted in much higher biomass and N uptake compared to enriched biochar. I demonstrated that the initial loading ratio was an important parameter that not only affected the nutrient removal efficiency from the liquid fraction of digestate, but also affected the availability of N and thereby the efficiency of the enriched material when used as a fertiliser.
In Chapter 6 (General Discussion), I synthesised the main findings of my research chapters and discuss their implications.
I concluded this thesis with a general conclusion in Chapter 7 (Conclusions and outlook) with a reflection on the findings of this thesis in the context of practical applicability of biochar and clinoptilolite for digestate treatment. I finalised this chapter with reflecting on the relevance of my main conclusions for future research.
In conclusion, this thesis confirms that sorption with clinoptilolite and biochar can be a promising technology to recover nutrients from liquid fraction of digestate. Using biochar and clinoptilolite provides concentration of nutrients and, thereby, volume reduction which allows for savings on storage, transport and application of the voluminous liquid fraction of digestate. The end-products are digestate-enriched clinoptilolite and enriched biochar can act as N fertilisers.
Original languageEnglish
PublisherDepartment of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen
Publication statusPublished - 2016

ID: 165179990