Guiding Principles in the Design and Operation of a Wastewater Sludge Digestion Plant with Biogas and Power Generation.

Background The South African industry is widely acknowledged for its excellence in process design. However, some disconnect have been identified between the work of the design engineer, the process manager and the process controller (Deacon & Louw, 2013). Opportunity presents itself to align the work of the Process Designer, who considers design criteria but often exclude operational optimisation of the system, and the Process Manager, who may lack design knowledge but are well conversed with operation of the plants. This study aims to not only align and optimise the process design and operation, but also to unlock the opportunities presented by integrated- and advanced sludge treatment methodologies. One of the value adds of sludge treatment is the generation of energy, which is gaining interest as the price of electricity increases and interrupted supply impact on the ability of treatment facilities to meet regulatory targets. Anaerobic digestion, coupled with Combined Heat and Power generation is becoming an attractive technology. The study explores the case of the City of Johannesburg’s full-scale CHP installation. Scope of Study This research study departs from the premises that: • sludge is a recoverable resource that can be developed; • sludge treatment is a consecutive and interdependent process which, if one process unit fails, the entire production chain is compromised; and • understanding the link between the design criteria, the operational criteria and performance measurement of each process unit, will build capacity on various levels. The purpose of the project is to: • provide a practical guideline for the design and operation of a sludge treatment plant, with enhanced CHP generation; and • identify and quantify the opportunities to replicate this approach across the South African industry, at municipal WWTW which already incorporate anaerobic digestion. Local and international references Interviews with professional engineers that design and construct sludge treatment facilities in South Africa, indicated that: • South African engineers do not follow any specific or prescribed local- or international design standard for wastewater or sludge process design. • Engineering design is typically based on ‘in-house’ preference and design criteria which are regarded as proprietary. • Larger municipalities tend to base their preference for design on previous experience with systems and processes that had, or not, worked in the past.