Question

Please assist. How is settled sludge handled in a rectangular clarifies, circular clarifies and clarifies for activated sludge.

Answer 1

Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. A clarifier is generally used to remove solid particulates or suspended solids from liquid for clarification and (or) thickening. Concentrated impurities, discharged from the bottom of the tank are known as sludge, while the particles that float to the surface of the liquid are called scum.

Handling of sludge in rectangular clarifier:

Non-metallic chain and flight systems having multiple flights (scrapers) installed at fixed spacing (typically 10 ft or 3 m) can accomplish both sludge and scum removal. The flights (scrapers) scrape the bottom of the tank and present consistent sludge flows to the hoppers. When the flights travel to the surface of the rectangular tank along the collector chain, the scum/foam is pulled by the flights to the scum trough. Removal of scum and floating solids is almost always easier with rectangular clarifiers, which are known to provide more effective scum/foam trappin g via rotating scum troughs.

Non-metallic chain and flight system for rectangular clarifier remove all sludge across the collector at constant speed of approximately 2 ft (600 mm)/min. the most negative aspect of rectangular clarifiers is the relatively frequent need to replace collector chains. Before the 1970s, the replacement frequency was every 10-20 years. After the implementation of non‐metallic chains, the frequency reduced to once every 15‐20 years. This timeline is in agreement with the commonality of the rectangular clarifiers.

Handling of sludge in circular clarifier:

In circular scrapper systems have only two rotating collector arms with multiple scrapers on each arm that plow the settled sludge to a hopper at the center of the tank. A single cantilevered skimmer arm rotates at the surface to collect scum and concentrate it in a hinged skimmer, from where it is then dragged up a scum beach and trough. Circular collectors move sludge at much faster rate at the perimeter of the collector than at the center of the collector. It is generally accepted that scrapper speeds greater than 6 ft (1800 mm)/min. can cause re-suspension of settled solids.

Handling of sludge in activated clarifier:

Activated sludge is also the name given to the active biological material produced by activated sludge plants. Excess sludge is called "surplus activated sludge" or "waste activated sludge" and is removed from the treatment process to keep the ratio of biomass to food supplied in the wastewater in balance. This sewage sludge is usually mixed with primary sludge from the primary clarifiers and undergoes further sludge treatment for example by anaerobic digestion, followed by thickening, dewatering, composting and land application.

Sludge treatment is focused on reducing sludge weight and volume to reduce disposal costs, and on reducing potential health risks of disposal options. Water removal is the primary means of weight and volume reduction, while pathogen destruction is frequently accomplished through heating during thermo-philic digestion, composting, or incineration. The choice of a sludge treatment method depends on the volume of sludge generated, and comparison of treatment costs required for available disposal options. Air-drying and composting may be attractive to rural communities, while limited land availability may make aerobic digestion and mechanical dewatering preferable for cities, and economies of scale may encourage energy recovery alternatives in metropolitan areas.

Thickening:

Thickening is often the first step in a sludge treatment process. Sludge from primary or secondary clarifiers may be stirred (often after addition of clarifying agents) to form larger, more rapidly settling aggregates.^[4] Primary sludge may be thickened to about 8 or 10 percent solids, while secondary sludge may be thickened to about 4 percent solids. Thickeners often resemble a clarifier with the addition of a stirring mechanism.^[5] Thickened sludge with less than ten percent solids may receive additional sludge treatment while liquid thickener overflow is returned to the sewage treatment process.

Dewatering:

Water content of sludge may be reduced by centrifugation, filtration, and/or evaporation to reduce transportation costs of disposal, or to improve suitability for composting.

Side stream treatment technologies:

Sludge treatment technologies that are used for thickening or dewatering of sludge have two products: the thickened or dewatered sludge, and a liquid fraction which is called sludge treatment liquids, sludge dewatering streams, liquors, centrate (if it stems from a centrifuge), filtrate (if it stems from a belt filter press) or similar. This liquid requires further treatment as it is high in nitrogen and phosphorus, particularly if the sludge has been anaerobically digested. The treatment can take place in the sewage treatment plant itself (by recycling the liquid to the start of the treatment process) or as a separate process.

Phosphorus recovery:

One method for treating sludge dewatering streams is by using a process that is also used for phosphorus recovery. Another benefit for sewage treatment plant operators of treating sludge dewatering streams for phosphorus recovery is that it reduces the formation of obstructive struvite scale in pipes, pumps and valves.

Digestion:

Sludge is often treated using a variety of digestion techniques, the purpose of which is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, and composting. Sludge digestion offers significant cost advantages by reducing sludge quantity by nearly 50% and providing biogas as a valuable energy source.