Frequently Asked Questions
The most common method to dredge a lagoon is an auger dredge. Auger dredges are typically cable driven that are anchored along the shoreline. The auger is usually 6 to 8 feet wide and turns to move sludge toward the center where the suction intake from the pump is located. Sludge is pumped via a pipeline to be dewatered in drying beds, belt presses, settling ponds, geosynthetic tubes, filter presses, or other dewatering means. The auger can be set to a specific depth or guided with wheels or guides to prevent damage to liners within lagoons.
You can measure the depth of water and sludge in a lagoon using a Sludge Judge®. A sludge judge will allow you to measure the depth of water down to the surface of the sludge and then push through it to actually sample the sludge as well. You can also use a rod or pvc pipe with markings to find the depth from the water to the top of the sludge. The depth of sludge is the difference in the top of the sludge and the firm resistance of the bottom (usually clay, liner, or concrete). A GPS or transect system can help you calculate the quantity of sludge in a lagoon.
Dredging is the method to remove sludge from a lagoon. If the lagoon can be taken offline and drained, then excavating the sludge may be an option. Otherwise, a hydraulic dredge can pump sludge from the bottom of a lagoon to remove it and restore the lagoon’s original depth.
Small hand operated dredges can be purchased for as little as $15,000, but they are only effective for limited types of projects and sediment. The smaller dredge barges can be purchased for $100,000 to $150,000. As you increase in size and effectiveness, most portable dredges cost between $500,000 and $2,000,000. The cost only increases for dredges bigger and with more features extending through the millions and for the largest dredges into the tens of millions. Auxiliary equipment and pipeline are an additional expense that needs to be considered as well in the cost of purchasing a dredge.
The time really depends on the amount of incoming sludge and capacity. However, most lagoons require dredging every 5 to 10 years on average. Some lagoons that treat or capture organic matter can be aerated and treated to extend the timeframe. Sludge depth can be monitored to provide data for timelines and planning of the next dredge.
Lagoons can be dredged to remove biosolids, alum, ash, sediment, or other sludge. Dredging of lagoons usually utilizes hydraulic dredging to pump the sludge for dewatering and disposal purposes. Lagoons are often a treatment for water quality to remove any contaminants, organic matter, or pollutants prior to discharge, reuse, or land application. If the lagoon loses enough capacity its function may diminish. Dredging can restore the function even by removing the sludge, often while the lagoon is still in use.