Screening of Filter Material With Weak Ammonia Nitrogen Adsorption Capacity, and Filtration Pretreatment of Suspended Solids in Biogas Slurry for Nitrogen Reuse

The key to improving the reutilization value of biogas slurry is to retain most of the slurry nitrogen content while removing the suspended solids (SS). For this study, static adsorption and desorption experiments were undertaken with quartz sand (QSD), a commonly used filtration medium, which was selected for its weak ammonia nitrogen adsorption capacity. A dynamic experiment was then carried out using two filter-filled columns with different QSD particle sizes to analyze the influence of QSD on SS and ammonia nitrogen in biogas slurry permeating through the column. Among the four low-cost filter materials tested (QSD, sea sand, zeolite, and alumina industry waste residue), QSD had the weakest ammonia nitrogen adsorption capacity in solution. The desorption rate of QSD was close to that of sea sand and far lower than that of zeolite and the alumina waste residue. In the QSD-filled columns an SS removal rate of up to 86.1% was achieved at a biogas slurry flow rate of 2.4 L·h^-1. The SS removal rate was generally higher in the 0.5-1 mm particle size QSD-filled column (QSD column) than in the 3-5 mm QSD column. However, there was no significant difference in the rate of ammonia nitrogen removal from the biogas slurry between the two QSD particle sizes used. After 24 h of operation, a blockage started to appear at the inlet of both of the columns, and after 32 h the 3-5 mm QSD column was almost completely blocked. The 0.5-1 mm QSD column required a longer running time and less filter material (a lower filling height) than the 3-5 mm QSD column to remove the same amount of SS from the biogas slurry. The SS removal rate of the 0.5-1 mm QSD column was more than 80%, and it had no significant effect on the ammonia nitrogen in biogas slurry. The 0.5-1 mm particle size was suitable for use in a pretreatment unit for SS filtration of biogas slurry and would be beneficial for any subsequent nitrogen recovery process.