Texas Waste Water Treatment #11 Sludge Digestion Dewatering & Disposal

The purposes of sludge digestion are to decompose (stabilize) enough of the organic material in the sludge so that it can be disposed of without creating a nuisance, to chemically separate the water from the solids, and to reduce bulk or volume, making the sludge easier to handle.

The types of sludge are Primary sludge (Primary/raw sludge refers to the suspended solids that have been settled out in primary sedimentation tanks or primary clarifiers.), Humus sludge (sludge that has been sloughed off from the zoogleal mass in a trickling filter and then unloaded to the final clarifier.), Activated sludge (sludge that settles in a final clarifier following the aeration tank and is continuously pumped from the clarifier. A portion of this sludge is returned to the raw sewage flow to seed the incoming material. The remainder, known as waste activated sludge, is usually conditioned and thickened before being pumped to an aerobic digester.), and Digested sludge (sludge that has been processed/digested to the point necessary for good drying).

The book says "Sludge conditioning is the process of removing water by thickening or by adding polymers or inorganic materials to improve the loading rate." this is almost as clear as mud and doesnt get a satisfactory explination or definition of terms in the book in my opinion. For the sake of understanding i would add that the goal of a waste water treatment plant is to separate recieved water from anything that might be in said water. At the same time we want to ensure that the substances removed are as inert as possible so as to make them safe for disposal. the processes we use to prepare the sludge for disposal (or condition the sludge) are designed to "stabilize" the sludge. the main goal of sludge stabilization is to reduce the amount of energy-rich, reactant, and easily degraded sludge compounds in the sludge so that once we put it in a landfill it is stable and doesnt emit gasses or change in volume or anything else. (for further information try googling "landfill leachate") This is what happens in the various biological reactions in the plant, and is what preceeds the solids treatment this question is asking about. A further aim of sludge stabilization is the decrease of high-water content, and which is what the question is specifically asking about. In this context we are refering to the addition of polymers or inorganic material to prepare the sludge for dewatering processes by making the solid mass stick together more a bit like adding more flour to runny dough or folding a towel before wringing it out. (The minimisation of the number of pathogens should be mentioned as a secondary goal of stabilization and not confused with the disinfection stage the water goes through. If we are going to put the remaining sludge in a landfill for example we dont want to risk introducing things that will make us sick into the environment.) Finally, the loading rate refered to here is talking about how much mass of sludge we can easily get into a given volume for the final dewatering process. To restate the original reference i would say "Sludge conditioning is a process whereby sludge solids are treated with polymers or inorganic materials to prepare the sludge for dewatering processes, in other words, to improve dewatering characteristics of the sludge."

The operation of sludge thickeners are influenced by, solids surface loading, constant aerobic conditions in the thickener, temperature of the sludge entering the unit, and biological activity of the sludge. "sludge thickeners" in this context refers to a holding tank and supporting equipment designed to encourage compaction of the sludge.

In the second stage of anaerobic digestion, the pH is raised to 6.8–7.4. Methane-forming anaerobic bacteria convert the volatile acids formed in the first stage of sludge digestion to methane and carbon dioxide, thus reducing the organic material in the system. The amount of methane produced determines the degree of stabilization. Methane-forming anaerobic bacteria are very sensitive to temperature and pH change and are much fewer in number than acid-forming bacteria. Because raw sludge has a low buffering capacity, fermentation produces volatile acids much faster than the few methane bacteria can consume them. The buffers are soon spent, and the existing free acids cause a low pH and a high volatile acids level (2,000–6,000 mg/L).

Methane-forming anaerobic bacteria convert the volatile acids formed in the first stage of sludge digestion to methane and carbon dioxide.

Anaerobic digestion is sensitive to sudden temperature changes. When the temperature of a digester is increased, no more than a 1°C change per day is recommended to avoid conditions which would interrupt or otherwise degrade operation.

The three common methods of mixing (or seeding) raw sludge into a digester are gas, mechanical (by propeller or draft tube), and pump. Gas mixing is accomplished by pulling the digester gas from the tank, compressing it, and discharging it back into the tank several feet below the surface. This gas rises to the surface through the digested sludge, creating a gas lift with a rolling action of the tank contents. Mechanical mixing uses propeller and draft tube propeller mixers. The propeller mixers are submerged 10–12 ft. in the sludge. An electric motor drives the unit. Draft tube propeller mixers are made of steel, with propellers 8–24 in. in diameter. The top of the tube is about 18 in. below the normal water level of the tank. The bottom of the draft tube may be straight or equipped with a 90° elbow placed so that the discharge is along the outside of the tank to create a whirlpool action. The propeller is about 2 feet below the top of the draft tube. A reversible motor allows sludge to be recirculated from the top down or from the bottom up. Pumps are sometimes used to mix the contents of small digesters that have heat exchangers. Either centrifugal or positive displacement pumps can be used. The pump may or may not be in the draft tube or at the bottom of the tank. Use of compressed air would add oxygen and make it a non-anaerobic environment.

A poor-quality supernatant liquor could be caused by low temperature, digester overloading, operating mixing devices during withdrawals, addition/withdrawal rates too high, and failure to withdraw digested sludge at the proper time.

Scum accumulation in a digester can affect operation and tank capacity. >Scum consists of sticks, hair, feathers, seeds, and undigested grease<, which together form a layer from a few inches to several feet thick. This layer obstructs the free flow of digester gas from below. Scum also can get into the area intended for supernatant liquor and interfere with withdrawal.

Production of digester gas will range from 1.0 to 3.0 ft.3/person/day varying with the type of wastewater treatment. The heat value of digester gas is from 550 to 700 btu/ft.3 . It can be used for digester heating, steam production, gas engine fuel, or space heating. The gas composition in a well-run digester will be about 65%–75% methane and 25%–35% carbon dioxide. When the gas is used, the collecting system should be equipped with safety devices, such as flame traps (to prevent backflash into the lines), pressure relief valves, condensate traps, and pressure regulators. if generated steam is used to generate electricity it could indirectly be used to cool buildings, you wouldn't want to aerate an anaerobic process, and the gasses are waste products from the digestion process and would not increase bacterial activity.

The gas composition in a well-run digester will be about 65%–75% methane and 25%–35% carbon dioxide.

Laboratory control tests can indicate digester problems, such as the following: Increased carbon dioxide content of the digester gas (30% of total gas production is normal) • Decreased total amount of gas produced • Increased volatile acids (50 mg/L minimum, 500 mg/L maximum) • Decreased pH • Decreased alkalinity (l000 mg/L minimum; 5000 mg/L maximum; 3000 mg/L normal) • Decreased supernatant quality • Decreased volatile solids reduction.

anaerobic processes would tend to be sealed in some way to prevent as much oxygen as possible from getting in and killing off anaerobic bacteria. conversely, an aerobic process would want to be open to the atmosphere so as to get as much access to the free resource as possible. Facultative bacteria can adapt to either condition.

A minimum value of 1.0 mg/L of oxygen should be maintained in the aerobic digester at all times to prevent filamentous and other undesired bacteria from developing.

Following digestion, the sludge is dewatered in preparation for final disposal. Methods of sludge dewatering in use are gravity thickening (depends on gravity to separate heavier solids from water. Solids settle and compact in the bottom of the thickener. The result is concentrated solids that reduce the amount of water that goes to final disposal.) • Sludge drying beds (water drains out through the sand and gravel of a pondlike container leaving the sludge behind to dry in the sun) • Belt presses (This process removes water from sludge solids by squeezing and compacting those solids into a sludge mat, or “cake,” that ranges in consistency from mud to wet cardboard.) • Centrifugation (The centrifuge is a spinning cylindrical drum or bowl. When a slurry is introduced into the interior of the centrifuge, it is thrown out against the bowl wall and forms a ring or pool where separation takes place similar to the spin cycle in your clothes washer at home) • Filter press (these separate liquids from solids by pumping conditioned sludge onto a filter cloth and applying pressure to the filter with two static plates. As liquid leaves the sludge, the liquid passes through the filter and is returned to the treatment system. When the plates are separated the sludge cake left on the filter drops out.) • Vacuum filtration (Vacuum filtration is a process of separating solids from liquids by passing the liquids through a porous medium on which the solids remain to form a cake.)

Drying time will be affected most by the degree of digestion of the sludge applied to your drying process. Other options listed below would affect different processes to differing degrees but the only option listed that affects them all is degree of digestion.

!!!!!!!!!!!!!!!!!!!!! The book specifically states "The depth of applied sludge should be determined (usually 8–12 in.)." and offers no further information. any comments on how that depth is determined or why that range is what it is would be greatly appreciated. !!!!!!!!!!!!!!!!!!!!!!!!

For sludge to be disposed of in a sanitary landfill, it must first be dewatered to prevent contamination of ground and surface water. In addition, the solids content must amount to at least 20% of the total.

Partially dewatered sludge can be incinerated. The small remaining amount of inert ash or inorganic matter offers no disposal problem. Temperatures of 1200°F (648°C) will burn domestic solids without producing odors. The greatest problem is the removal of the ash from the unit. Incineration is an expensive method of sludge disposal. It is usually not feasible at small treatment plants and is not advisable at any plant where alternative disposal options are available.