Industry Trends Winter 2006

Many sanitary districts may have to perform weekly cleaning on troublesome areas within their collection systems without questioning the reason for the weekly ritual. More commonly, collection systems that have a history of backing up require more maintenance, such as cleaning, especially during the wet weather season. Over the past year, V&A has become aware of several cleaning methods that remove large amounts of sediment from pipelines more efficiently than a standard positive displacement sewer cleaner truck. Based on V&A’s familiarity with some of the technology, we would like to share this information with our readers.

Sediment Buildup

After many years in service, sewer collection systems accumulate sediment in the flow channel. The sediment buildup is particularly more pronounced in segments of pipe where the flow velocity drops below 1 foot per second, such as upstream and downstream of a siphon. Other areas of sediment buildup include segments with insufficient slopes, junction structures with poor flow hydraulics, or rapid expansions in the pipe diameter. Photos 1 and 2 illustrate some of these conditions.


Photo 1. Stagnant flow and debris surcharge the manhole. .	Photo 2. Flow capacity is limited by high sediment builldup in the pipeline.

Solids Removal Options

There are several solids removal/dewatering systems available throughout the United States, however there are not many applicable systems for removing sediment from large sanitary sewers (i.e., 48-inch to 120-inch diameter pipelines). The following list provides an overview of the types of systems available and their applications.

Sludge pump immersed in a sanitary sewer pipe discharges into a dewatering bin with a permeable liner that captures all of the large solids.
Sludge pump immersed in a sanitary sewer pipe discharges into a truck mounted bin that captures all of the large solids.
Vactor Truck inserts a vacuum hose into the sanitary sewer pipe and removes the sediment and discharges it into a truck-mounted bin.
Addition of a flocculation polymer prior to a belt press which mechanically dewaters biosolids from lagoons or digester sludge and creates a dry sheet that can be disposed of at a landfill.

The use of a dewatering bin (Photo 3) is an effective means of removing silt and debris from a sewer pipeline, however it may require excavation for bypass pumping and a staging area for the bin. It does not utilize high pressure water jetting to move the sediment to the sludge pump. Photo 4 shows the water jetting hose used to move the sediment to the sludge pump and the dewatering bin used to store the solids. This method has been used extensively to remove large amounts of sediment and debris from sanitary sewers. Photo 5 shows one step in the process of dewatering the solids extracted from a lagoon or digester. To date, V&A is not aware of this process being used in sanitary sewers. Photo 6 shows the Vactor truck’s boom hose being lowered into the manhole. Some trucks are equipped with a water jetting pump that is inserted into the pipeline to clear blockages. Vactor trucks are effective for small amounts of debris or sediment in blocked pipelines, however it is not cost effective for large amounts of sediment or debris.

V&A has conducted physical inspections to determine the amount of sediment in flow channels and has assisted in planning of the disposal of sediment. The recommended removal method will depend on the amount of sediment in the pipeline. V&A does not haul or remove the sediment from the sanitary sewers. Contact V&A at the number below if any further information or guidance is needed with respect to these services.