Determination of Chemical Oxygen Demand in Substrates from Anaerobic Treatment of Solid Organic Waste

Abstract

A modified method is described for the determination of the chemical oxygen demand (COD) of samples with high solids content, especially those used in biogas production. The modified method is based on ISO 6060 and DIN 38414 methods. Analyses with potassium hydrogen phthalate solution standard showed that the modified method is valid and gives equal results to standard method (9,960 mg/L determined by ISO method, and 9,930 mg/L by modified methods). Comparison of the standard method with the new proposed method using actual samples shows that the modified method gives more reliable results than the standard method. The standard ISO 6060 method is more appropriate for samples with homogeneous solids, where uniform dilution is easy to achieve. Although the modified method shows slightly more accurate results, the increased chemical expense associated with the modified method does not justify its use. The proposed modified method is especially appropriate for wet solid samples with high total volatile suspended solids content and the COD values of up to 400,000 mg/kg. COD for waste activated sludge determined by ISO 6060 method was 47,300 and 52,800 mg/kg determined by modified method. COD of original wet solid sample was 360,000 mg/kg, and COD for previously dried samples at 105°C was 345,000 mg/kg. For anaerobically digested sludge the COD was 80,800 mg/kg, and COD for previously dried samples at 105°C was 72,700 mg/kg. The difference is attributed to the loss of volatile compounds in drying. Applying the modified method, samples need no preparation and are analysed in their basic wet form as they are collected from anaerobic digesters. Analysis has shown that currently available methods for COD determination with samples containing high amounts of solids require drying of the sample with the loss of volatile components. This may lead to inaccurate results.