Sludge Drying Beds
The use of sludge drying beds is usually limited to small to medium sized (< 5.0 MGD) plants due to land and manpower requirements. In the southern states, the abundance of warm weather, sunshine, wind and land makes drying beds a good sludge drying process for many treatment plants. In the northern states, this is not always the case. Sludge drying beds can be divided into three categories;
(1) sand drying beds,
(2) asphalt or concrete drying beds, and
(3) vacuum filter beds.
Sand drying beds consist of shallow lined concrete or earthen basins with perforated pipe installed under a 12 – 18 inch layer of gravel with an 8 – 12 inch layer or sand placed at the top. Decant tubes are located in the corners of the bed to drain any pooled water off of the sludge in the bed. Sludge is poured on the bed to a depth of around 12 inches. Water evaporates from the sludge as well as percolated down through the sand layer and is collected and removed by the perforated pipe under-drain system. When the sludge in the bed has dried to the point that it has cracked all the way down to the sand, the sludge is removed using a shovel and wheelbarrow or a small skid loader. Many older drying beds cannot be cleaned with a skid loader because the under-drains system will be crushed.
The factors that most affect the performance of sludge drying beds are:
1. Climatic Conditions. Little can be done by operators to change climatic conditions. Some sludge drying beds remain frozen throughout the winter and can only be cleared after they thaw and dry out. Rain and snow are less of a detriment to sludge drying than freezing conditions, because dry or partially dry sludge does not take up much water.
2. Depth of Sludge Pour. Sludge should not be poured much deeper than 12 inches because the drying time will increase substantially. Sludge should never be poured onto a drying bed that already contains partially dry sludge. This practice is known as “capping” and should be avoided because the lower layer of sludge will get sealed off. This lower layer becomes what is called “green sludge”, which is extremely odorous and will not dry.
3. Condition of Sand. The sand should be carefully leveled before sludge is applied to a sand drying bed in order to avoid areas that dry more slowly than the rest of the bed. Older drying beds can become compacted and the sand will not permit water to permeate. If this happens, the best cure is to remove 2 – 3 inches of sand and replace it with fresh, pre-washed sand. Some drying beds use expensive screened and pre-washed sand while others work fine with what could best be described as “arroyo sand”. The best sand has no dirt or clay in it and is free of excessive fines, which are very small particles.
4. Use of Polymers. Polymers are often used to improve the performance of sand drying beds. If used properly, polymers can cut the sludge drying time in half. This has the effect of doubling a plant’s number of drying beds.
Sand drying beds are capable of drying sludge to > 95% Total Solids, but 70 – 80% is more typical.
Asphalt drying beds are similar in construction to sand drying beds, except that instead of sand and gravel they have a hard asphalt or concrete surface. The hard surface allows the use of sludge mixing equipment to speed the drying time and skid loaders to remove the sludge when it is dried. Decant tubes are an important feature of asphalt beds because pooled water would have to evaporate (taking too much time) if it could not be decanted off. Sludge can be poured to a greater depth (18 – 30 inches) in asphalt beds because the action provided by the mixing equipment will help it dry rapidly. Mixing can be accomplished using an ordinary tractor, backhoe or dedicated sludge mixing equipment like the units manufactured by Brown Bear™ and others. Asphalt beds have proven very successful in dewatering sludge prior to composting, especially in the southern states.
Vacuum filter beds offer a relatively new method for dewatering sludge. Vacuum filter beds consist of a shallow concrete basin that has an under-drain system covered over with porous pumice bricks or stainless steel or plastic perforated panels. Sludge that has been conditioned with polymer is poured onto the bed and a vacuum pump is used to create a vacuum underneath the panels. The vacuum rapidly draws water from the sludge. Vacuum beds can dewater sludge to 15 – 30% TS in a matter of hours or sometimes days. The bed is then cleaned out using a skid loader or small backhoe fitted with a front-loading bucket. Some systems using stainless steel or plastic panels, but without a vacuum pump system, are also in operation.