Primary and digested sludge-derived char as a Cd sorbent: feasibility of local utilisation

Area of Science:

• Environmental Science
• Materials Science
• Chemical Engineering

Background:

• Cadmium (Cd) is a toxic metal pollutant found in municipal wastewater and industrial effluents.
• Sewage sludge char presents a potential low-cost adsorbent for Cd removal.
• Assessing local char production feasibility and Cd sorption capacity is crucial.

Purpose of the Study:

• To evaluate the Cd sorption capacities of chars derived from primary (PSC) and secondary sludge (DSC).
• To assess the feasibility of local char production for Cd sorbent purposes.
• To analyze the pyrolysis energy balance for sludge char production.

Main Methods:

• Pyrolysis of primary and secondary sewage sludge at varying temperatures.
• Cadmium sorption experiments using synthesized chars.
• Analysis of regional Cd sorbent demand and char production economics.
• Evaluation of the energy balance of the pyrolysis process.

Main Results:

• Primary sludge char (PSC) exhibited superior Cd sorption capacity (9.1 mg/g) compared to secondary sludge char (DSC) (6.0 mg/g) at 800 °C.
• Sorption capacity increased with higher pyrolysis temperatures.
• Primary sludge pyrolysis showed a more favorable energy balance, but became negative when biogas production loss was considered.
• Local Cd sorbent demand could be met, but required large char volumes, limiting feasibility.

Conclusions:

• Sewage sludge char is a viable, low-cost option for cadmium removal from wastewater.
• Primary sludge char demonstrates higher efficiency, but production scale and energy balance require careful consideration.
• The large volumes of char needed for regional demand present a significant challenge to the widespread application of this method.