Solving operational problems with inverted siphons

Inverted siphons on sewer networks typically suffer three main types of operational problems: siltation, air locking and odour. This paper describes the successful implementation of solutions for all 3 of these problems at inverted siphons on North Shore City Council’s (NSCC) trunk sewer network.

Siltation and blockage is often thought of as the main operational problem likely to afflict an inverted siphon. NSCC has one trunk sewer with 4 single barrel inverted siphons in series, which has had a history of overflow problems. These inadequacies have now been mitigated by the construction of a storage tank upstream and the addition of access and drainage facilities.

Air Locking is another cause of reduced siphon capacity, and is usually the result of poor hydraulic conditions at the inlet and insufficient consideration of air passage out of the descending leg. NSCC has a twin barrelled siphon that has suffered particular air locking problems until recently. These problems were solved using a low-cost venting arrangement on the descending leg, avoiding the need to replace sections of siphon barrels with larger diameter pipes.

Odorous air discharge has been the mo st frequent and challenging problem encountered by NSCC. Biofilters have been one of NSCC’s favoured solutions for odour treatment, and 2 have been constructed at siphon locations, with good, although varying, levels of success. However, a combination of odour treatment together with chemical dosing is currently being trialled, and is showing considerable promise.