Sludge Handling Post-Earthquake - The Christchurch Wastewater Treatment Plant Story

During the Canterbury earthquake sequence, the digesters were damaged to varying degrees and in a variety of ways. Fortunately, they were able to keep operating. The four mesophilic digesters have since been taken out of service one at a time, for clean-out and repair. In doing so, it was discovered that they were 80 to 95% filled with sand, yet they had continued to produce biogas. Also, although all four digesters are the same size and type, they have some design differences, reflecting staged installation over a number of decades that have affected their response to the earthquakes. The rapid sequence of work has provided a unique opportunity to identify these commonalities and differences in damage and repair requirements.

This paper will describe:

• Why and how the solids train as a whole continued to operate and potential consequence of a failure
• The different types of damage to the digesters and the repair methods employed
• The impact of the influx of sand
• Lessons learned