GAC filter Design Criteria for Wastewater Treatment for Removal of Organic Micropollutants – A Literature Review

Abstract: Granular Activated Carbon (GAC) is nowadays suggested as a fourth (quaternary) treatment step at municipal wastewater treatment plants to remove certain chemicals and particularly organic micropollutants (OMPs). This work investigated the different factors that affect the performance of the GAC filter for the removal of micropollutants in a municipal wastewater treatment plant. This study is purely based on compiling data and analyzing different studies. Carbamazepine, diclofenac and sulfamethoxazole were chosen as breakthrough indicators. The comparisons were made by plotting bed volumes versus dissolved organic matter (DOC) and bed volumes versus empty bed contact time (EBCT) for a breakthrough criteria of 20%. A breakthrough occurs when the filter attains a breakpoint as saturation of OMPs, and the organic matter occurs. At that point, the filter cannot achieve the standard removal efficiency anymore. The pollutant concentration will then increase in the filter effluent after this breakpoint. High pH makes the contaminants negatively charged, and activated carbon is also negatively charged, which causes repulsive force between the activated carbon surface and the OMPs. The increase in pH consequently decreases the adsorption of OMP's. The molecules are more stable at lower temperatures and tend to be adsorbed quickly. pH and temperature are essential for biological processes in the wastewater treatment plant. pH is recommended to be at a neutral phase of 6.5-7.5, and temperature is recommended at 20 – 35 0C. These conditions are perfect for organisms to thrive. Bitumen, lignite, coconut-based GAC did not show any significant difference in performance, which can be explained due to lack of data where characteristics of influent and pilot setups were diverse from the collected studies. EBCT is an essential factor for the removal of OMPs. Approximately 20 – 30 min is required for typical wastewater treatment and more than 30 min for physico-chemical where chemicals like coagulants and flocculants are used in wastewater treatment. Sulfamethoxazole showed higher resilience towards adsorption and required higher EBCT compared to carbamazepine, diclofenac. SS and DOC play an essential role in predicting the bed life of the filter, where high DOC and SS can cause fouling (clogging of pores) through the accumulation of particulate matter by SS and buildup of biomass by DOC. The suspended solids content is recommended to be as low as possible, and DOC of less than 10mg/l is recommended for GAC filtration. The carbon usage rate for typical biological wastewater treatment is at 0.1 – 0.21 kg/m3 due to less use of chemical treatment, and for physico-chemical treatment, it is 0.21-1.04 kg/m3 where chemicals such as alum and iron salts are used for coagulation and flocculation to form flocs. Based on materials such as bitumen, lignite, and coconut, for US mesh size at 8x30, the effective grain size was found to be similar for all the materials at 1.25mm.