Abstract
Aluminum sulphate (alum, Al2(SO4)3·nH2O) has successfully been used to reduce ammonia loss from poultry litter, cattle feedlots and manure composting, but has not yet been utilized in the thermal drying process of digestate solids. The objectives of the present study were to evaluate the effects of alum addition on ammonium nitrogen (NH4+-N) content and phosphorus (P) solubility in dried digestate solids in comparison to the addition of concentrated sulfuric acid (H2SO4). Manure-based (MDS) and sewage sludge-based (SDS) digestate solids were chosen to conduct a drying experiment at four pH levels (original pH, 8.0, 7.5 and 6.5) and using two acidifying agents (alum, concentrated H2SO4). Alum addition increased the final NH4+-N content significantly from 1.4 mg g−1 in the non-acidified control up to 18 mg g−1 and 10.8 mg g−1 in dried MDS and SDS, respectively, which were higher levels than obtained with the addition of concentrated H2SO4. Moreover, alum considerably lowered the water extractable phosphorus (WEP) in raw and dried SDS by 37–83% and 48–72%, respectively, compared with the non-treated control. In contrast, concentrated H2SO4 notably increased WEP in raw and dried MDS by 18–103% and 29–225%, respectively. The comparison between the two acidifying agents indicated that alum had the potential to be an efficient and easy-handling alternative to concentrated sulfuric acid, resulting in higher NH4+-N content and lower P solubility.
Highlights
Anaerobic digestion is a well-established waste-to-energy technology for treatment of various organic wastes, which facilitates recycling of valuable nutrient resources like nitrogen (N) and phosphorus (P) in the waste materials [1]
Two kinds of digestate solids derived from anaerobic digestion of either manure (MDS—70% dairy, 20% pig, 8–9% chicken manure and 1–2% food waste) or sewage sludge (SDS—the secondary sludge after chemical removal of P using ClFeO4S and sludge precipitation with AlCl3) were freshly collected from Morsø biogas plant (Mors, Denmark) and Bjergmarken wastewater treatment plant (Roskilde, Denmark), respectively
According to the pH evolution test of MDS and SDS for 72 h after alum addition (Figure 1), the pH decline could be divided into three stages: I, precipitous decline stage, instantly after alum addition; II, slow decline stage, 0–24 h, and III, relatively stable stage, aifntestra2n4tlyh aufpteornaaludmditaidodni.tIionnM; IDI,Ss,lomwodstecolfintheestpaHge,d0e–c2li4nhe,(afrnodmII9I,.1retloat7iv.9e)lyhasptapbelenesdtagine, sataftgeer I2,4ahnduppoHnraedmdaiitnioend. aInt aMroDuSn,dm7o.8staoftfetrh2e4phHudpeocnlinaded(firtioomn.9F.1ortoSD7.S9,) phHapdpreonpepdeidn fsrotamge8.I6, atond7.1pHin rsetamgaeiInaenddagt raardouuanldly7d.8ecalfitneerd2t4oh6.u4pinonstaadgde iItIi.oTnh. eFroefroSrDe,SM, pDHS adnrdopSpDeSd wtfStriraeDtotrrimSaeotwnhio8oec.n6murecrtouvohgreo7vemmn.e1eoemoiganueseausnslstreyaueogmrmueesmeilIxnyeeatn.dmntd.wixgeitdrhawdaluiutahmllayalundmdeckalinenpdetdkaettporto6ao.t4mriontoesmmtaptgeeemraIpIt.uerTraehtueforreref2foo4rreh,24MbehDfobSreeafontird-e
Summary
Anaerobic digestion is a well-established waste-to-energy technology for treatment of various organic wastes, which facilitates recycling of valuable nutrient resources like nitrogen (N) and phosphorus (P) in the waste materials [1]. Up to 95% of the NH4+-N contained in the solids can be emitted as NH3 during the drying treatment [4] due to the high pH (>8.0) of the digestate solids and the high temperature (>70 ◦C) of the drying process. To avoid these massive N losses and improve the fertilizer value of the dried products, concentrated sulfuric acid can be used to acidify the digestate solids before drying [4]. Roboredo et al (2012) [5] and Sommer et al (2015) [6] reported that acidification using concentrated sulfuric acid increased water soluble P in the solid fraction of pig slurry, resulting in a higher risk of P leaching and runoff
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