Water Industry News

Sludge Processing

By Roy Bigham
Sludge!! The word aptly provides a mental picture of material generated from many industrial or municipal water treatment systems. In most water treatment systems, water is treated to precipitate contaminants. At this point, the water is usually cloudy and contains about 0.5 to 3 percent solids. The sludge is then thickened to improve settling, and pumped to a press or drier system. After the drying process, the sludge is often called a cake because the consistency has changed with a solids content anywhere from 30 to 90 percent. Here is a short discussion of just a few of the processes that handle sludge and reduce disposal costs.

Sludge thickening

Figure 1. The above flow diagram demonstrates the flow under a baffle and up through a sludge blanket. The blanket helps filter solids from the water.
The sludge material often needs to be brought together to form larger precipitates to allow it to efficiently settle. There are a number of processes to accomplish this task, and it depends on the characteristics of the material to determine what would work best in each case.

Occasionally, gravity alone will do the job. A dilute sludge solution flows under a baffle and up through a sludge blanket – a layer of sludge lying on the bottom of a container (see Figure 1). As the sludge in solution contacts the sludge blanket, it is then filtered out and the blanket grows. The sludge is regularly pumped out of the bottom so as to maintain a certain thickness for the sludge blanket to maintain proper filtration. If the blanket becomes too large, it can restrict flow or channels can form, reducing effectiveness.

Another method of sludge thickening is to add a coagulant. These are special chemicals added to dilute solids solutions before the sludge blanket. Some mixtures contain solids that carry a charge that prevents the solids from joining together, thereby preventing adequate precipitation. The coagulant coats the particles, allowing them to come together. There are various materials – usually large, carbon-chained chemical compounds – that are used as coagulants. The materials must be very carefully mixed. Improper mixing will either form so-called “eyes,” or break the large chains making the solution ineffective. Eyes are lumps of material that have water attached to the outside but the middle of the lump is still dry. It is advisable to contact a number of suppliers before choosing a coagulant. They can save a lot of time and money by providing help in the selection process and provide guidance on proper mixing. Suppliers will often provide a number of samples to test with each treated water system to find the most effective product.

Whether a gravity thickener or a coagulant is used to thicken the sludge, a proper thickness of the sludge blanket must be maintained.

Paddle dryers provide improved surface contact to efficiently dry sludge with indirect heat using a variety of mediums. Photograph supplied by Komline-Sanderson.

Often, routing the flow through a sludge blanket will not be sufficient by itself to remove enough solids to reach discharge requirements. Some type of settling tank must be supplied. The idea is to slow the flow sufficiently to allow solid material in suspension to settle. If there is sufficient space, a settling pond is often employed. Filters are another option; many are self-cleaning or easily backwashed to maintain efficiency. If space is a concern, a slant plate clarifier can be employed. This equipment allows the flow to be slowed and the design allows the sludge particle to drop about 2 inches instead of many feet, as would likely be needed in a pond.

After the sludge has collected on the bottom of a tank, it must be removed. Again, there are a few choices. Tanks with flat bottoms use a slow-moving rake to collect the sludge in a sump. Some tanks are designed with sloped bottoms, shaped like a cone or an upside-down pyramid. Sludge is often pumped using a diaphragm pump. If a pond is used, the sludge is removed by mechanical digging.

The sludge at this point appears as a thick, goopy material, while moisture content can vary widely, depending on the source. There is more water in the sludge than is often assumed. It is likely to be less than 10 percent solids.

If the treatment system uses bacteria in the process, it is possible to reuse a portion of the sludge to save money. Often, the sludge will contain healthy bacteria and an amount of the sludge can be pumped back to the beginning of the process to lower the cost of bacteria additions. Care must be exercised if the sludge is pumped back to the process. The bacteria must be healthy and the correct type or the treatment system will be upset and must be restarted.

Pressing or drying

Gravity belt thickeners can quickly thicken sludge to a range of 5 to 8 percent in a small space. The model G-25 by Komline-Sanderson shown above is rated to treat up to 120 gal/min with 25 square feet of drainage area. 

Photograph supplied by Komline-Sanderson.

With moisture content exceeding 90 percent, it is not cost effective to move the sludge to disposal. Again, there are a number of choices to consider in removing the water. There are drying ponds, filter presses, rotary furnaces and paddle dryers to name a few.

The least complicated would be a drying pond. Generally, at least two are needed depending on size, volume, weather and turnaround time. For this exercise, assume two ponds are required. One pond is filled and allowed to bake in the sun. While one pond is drying, the other is filled. Before the second pond is filled, the first one is excavated using digging machines. Then the process reverses. The amount of moisture removed is dependant on holding time, weather and the sludge’s physical characteristics.

Filter presses are filled directly from the sludge collection tanks. The presses are usually made up of a series of concave plates with special fabric covers that act as filters. The space between the plates is filled with the sludge. Hydraulic or pneumatic pressure then squeezes the plates together to remove the moisture. The plates are then separated and the resultant cake is allowed to drop into a container. Depending on the characteristics of the sludge, this can produce a cake that is 60 to 90 percent solids. The number of plates can be increased as needed or multiple filters can be run in parallel as volume dictates. Presses generally require less space than ponds.

A rotary furnace dries the sludge by fanning flames and hot air across the sludge. Occasionally, a rotary furnace is used in conjunction with other drying methods to reach a desired dryness specification.

A paddle dryer is an indirect heating process. It is a long tank with paddles that rotate through the sludge. The paddles are often heated with steam, water, oil, glycol or thermal fluids. The heated paddles drive the moisture away while breaking up the clumps and moving the material through the process. Some sludge can form a very tough crust that makes driving the moisture off with other methods very difficult. The paddles will break up the sludge, allowing more exposure to the heat source.

As mentioned at the beginning, these are a few of the methods for handling sludge. Decisions must be based on the characteristics of the sludge produced by a given process, the space available, turnaround time required and more. PE