Chemical stabilisation of anaerobic digestate via wood ash-based treatment

Zusammenfassung

The depletion of world´s mineral resources increases the cost of production of chemical fertilisers for agriculture, due to the need for extraction of raw materials in poorly accessible and inconvenient sites. An enhancement of the circular economy by means of reprocessing clean nutrient-rich waste materials will also address environmental challenges associated with the low use efficiency of the nutrients, which leads to pollution of water, soil and air due to the excessive fertilisation of fields to maintain crop yields and provide enough foodstuff. This thesis investigates the treatment of anaerobic digestate with wood ash and commercial acids (i.e. sulphuric, hydrochloric, nitric and lactic acids) for the preparation of a stable slow-released fertiliser and mineral products, such as ammonium sulphate. The extraction of the biogas from the anaerobic digestate implies the mineralisation of some organic nutrients (e.g. organic nitrogen; Norg), thus becoming more available for plants (e.g. formation of ammoniacal nitrogen; NH4+-N). The application of this material to land represents a problem due to the volatilisation of ammonia and leaching of nitrate (after nitrification of ammonium in the soil) and phosphate. The role of the wood ash as sorbent could be used to control the availability of the nutrients already present in the anaerobic digestate. Moreover, the wood ash provides additional phosphorus to generate a more balanced organic fertiliser, according to the type of crop. The investigation of the wood ash treatment of the anaerobic digestate under different pHs was necessary to find the best blending conditions of these two waste streams. A food waste digestate (FWD) and a post-harvest vegetable waste digestate (PVWD) were the two types of organic material considered. A wood fly ash (WFA) and a wood bottom ash (WBA) were the two types of sorbents employed and they were produced in the same combined heat and power plant. Among these samples, the best controlled-release soil organic amendment was found to be produced by blending the PVWD, which has a lower ratio NH4+-N/Norg, and the WFA, which is the finer fraction of the ashes with more black carbon content and surface area to enhance the sorption processes. The pH of zero charge of WFA (11.90 ± 0.50) was found to be the optimum and it could be attained by mild acidification with HCl, maintaining a blending ratio lower than 3 g total solids (TS) WFA/g TS FWD or using non-invasive H2SO4 acidification (i.e. closed chamber) to recover the NH3 volatilised. The solid-liquid separation of the blend of wood ash and anaerobic digestate was performed as part of the analyses of the availability of the nutrients (N, C and P). Nevertheless, a more detailed study needs to be conducted on whether this share of wood ash is enough to produce a granular fertiliser with sufficient mechanical properties (e.g. durability) for packaging commercialisation.