In this section, several case studies are described that relate to common issues, occurrences, and process failures encountered in the field. 

These case studies are based on actual process enquiries, field technical support incidents, and process upgrade and commissioning projects in the UK.

Enquiry for technical support: 

The personnel attending the process noticed fine sludge and pin floc rising in the FST and fine foamy solids floating on the FST surface. The process effluent was deteriorating steadily over several weeks, with poor SSVI measurements and FE discharge suspended solids value creeping up to the permit limit.

Site actions: 

Attending personnel switched between the two available FSTs and over-pumped effluent back to the oxidation ditch to prevent the process from failing during an imminent scheduled environmental sampling procedure.

Process assessment:

The process was assessed with the following steps:

•  Review the installation’s design, assets and process technology.

•  Discussion with attending personnel about the problem. 

•  Reviewing historical data from the past two to three months. 

•  Inspecting the process installations from the FE discharge back upstream to the inlet. 

•  Discussion with regional technicians and decision makers.

Relevant findings reported

•  The FST mechanisms, blanket and WAS systems settings were functioning with no obvious faults, loading concerns or visible blockages.

•  The hydraulic retention time of the main aeration reactor for the prevailing average flow was determined to be 9 hours.

•  The main aeration ditch was a completely mixed, racecourse-style system with automated surface aeration.

•  The aeration ditch mechanical systems and aeration systems were functional and operating with DO control according to the correct range settings. (1.0 – 2.5)

•  The aeration system was controlled with timers to switch the aeration off in the high-demand charge period from 16:00 to 20:00 daily. (4 hours)

•  The final effluent quality was failing in BOD, NH4 and SS for 4-8 weeks before the sample was taken by the FE monitoring team, with a sample scheduled in the late evening (22:00).

Technical explanation: 

Switching off aeration on oxidation ditches is commonly practised, provided that the anoxic period does not exceed 30 to 40 minutes for the BOD treatment to continue utilising the nitrate present in the bulk water. Under anoxic conditions, further nitrification is impaired, and ammonia treatment is interrupted. Because the system is completely mixed, the loss of ammonia treatment capacity with no aeration is immediate. Switching the aeration off for 4 hours a day will halve the BOD and ammonia treatment capacity immediately, and over an extended period, cause the large sludge floc to break up and disperse.

Root cause: 

Operating the aeration reactor for only 5 hours per average HRT cycle, switching the aeration off completely for 4 consecutive hours.