Comparison of Alum and Sulfuric Acid to Retain and Increase the Ammonium Content of Digestate Solids during Thermal Drying

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.