Sanitary Sewer Operations: Blockage Removal

By NASSCO member Stephen Tilson, Collection Systems Operations Consultant, Tilson Associates | June 2010 Vol. 65 No. 6

Of all sanitary sewer operations, blockage removal is perhaps the most demanding and highly visible. For the purpose of this discussion a blockage is defined as the “stopping or interruption of sanitary sewerage flow.”

These can be temporary events that stop flow for a short time, allow the “weeping” of water to filter through solids, or it can cause a sewer spill when water backs up and relieves at the lowest point in the system via manholes, cleanouts, or plumbing systems inside structures.

Blockages & Safety
Blockages fall into five primary categories: Blocked flow with manhole(s) holding water but not spilling; Spilling but contained/ponding on its own; Spilling and relieving into waterways; Spilling inside a structure, and; Large diameter pipe/forcemain spilling requiring by-pass. Outside of the local regulatory agency reporting criteria, there are elements of tactical response and remediation that should be observed.

From a safety standpoint, blockages should be relieved from the downstream, dry manhole. Many serious accidents have occurred from using high velocity flushing machines from the full manhole. If relieving the blockage from the full manhole is the only option, extreme caution should be used when flushing equipment is used in this situation.

Relieving blockages
Relieving blockages can be quick and easy, or difficult and slow depending on circumstances. The key is patience in working the high velocity jet nozzle. Selecting the proper tool is critical, nozzles specifically designed for this task such as penetrator and/or chisel point should be utilized. They will provide the thrust required to break through and break up the blockage.

Blockage removal is typically achieved by working the nozzle into or over the blockage. The key is to keep the nozzle moving at all times. In order to avoid and/or minimize damaging the pipe, do not to leave the nozzle in one position for extended periods. Once the nozzle passes through the blockage, a steady hose rewind rate of less than one (1) foot per second should be maintained to ensure effective cleaning of the blockage material.