Adsorption treatment of sewage sludge from heavy metals

Abstract

Introduction. Because of urban development the volume of municipal and industrial wastewater are growing. Along with them the amount of sewage sludge (SS) also increases. Millions of tons of SS are currently accumulated on the territory of the Russian Federation and contain various pollutants, including heavy metals (HM). In this regard, the search for effective methods of SS treatment from HM is an urgent problem. The main methods of SS treatment are drying, dehydration, thermal methods, UV and microwave wave treatment. These kinds of disinfection eliminates many pathogenic microorganisms, but they are quite expensive and not effective against HM. Reagent methods include SS disinfection with quicklime (CaO). However, decontamination requires large doses (up to 30%) and it is also ineffective against HM. Humic-mineral reagent are more effective, they are based on crushed caustobiolites, their cleaning capacity from HM is 19–87%. Methods and materials. The authors have previously shown the effectiveness of wastewater treatment from HM using sorbents based on dolomite, quartzite, and waste from mining and processing plants. Therefore, a method for SS treatment from HM using sorbents based on dolomite, humates, and CS containing CaCO3 and humic compounds was proposed. In this regard, a method was proposed for SS treatment from HM using three types of sorbents based on: 1) waste of thermal power plants (TPP) – conditioned sludge (CS) containing CaCO3 up to 68% and humic compounds up to 12% – sorbent 1 (S1); 2) dolomite – Mg and Ca carbonate in a composition with sodium humate (25%)– sorbent 2 (S2); 3) modified dolomite with sodium humate (1%) – sorbent 3 (S3). Results and discussion. In laboratory experiments, the cleaning capacity of SS was studied using a dolomite-based sorbent modified with humate (1%). In field tests, a decrease in the concentration of HM in SS was studied with the use of sorbents based on CS and the complex sorbent dolomite-humate (75:25). The cleaning capacity of SS from HM increases in the series: sorbents based on waste from TPP – CS containing CaCO3 and humates (cleaning capacity E = 4.8–48.6% for dried SS and 29.3–53.3% for dehydrated SS) < sorbent based on a composition of dolomite with humate (E = 65.1–92.1% for dried and 56.6–89.4% for dehydrated SS) < a dolomite-based sorbent modified with humate (E = 90.8–99.9%). Conclusions. The maximum cleaning capacity is shown by a dolomite-based sorbent coated with a nano- and micro- sized layer of sodium humate.

Keywords

• sewage sludge treatment
• sorbents
• heavy metals
• dolomite
• humate