Texas Basic Waste Water Operations Ch#11 Sludge Processing

One of the last steps of wastewater treatment is digestion of the solids removed. There are four primary purposes of digestion, that include the following: • Reduce sludge volume • Make sludge easier to dewater • Stabilize organic solids • Reduce sludge odor

The following types of sludge are the results of treatment: • Primary sludge • Humus sludge • Activated sludge • Digested sludge

Primary sludge is raw sludge and settles in the primary clarifier. Primary sludge has these characteristics: fecal odor, composed of 94–96% moisture, does not dry readily, and usually is digested before disposal.

Humus sludge is sloughings from the zoogleal mass in fixed-film processes that settle out in secondary clarifiers.

Activated sludge settles in the >final clarifier< or is wasted from an activated sludge process. Activated sludge is about 99% moisture and may be digested before disposal.

Digested sludge is necessary for good drying. Sludge is digested aerobically or anaerobically.

Aerobically digested sludge has the following characteristics: brown color, very little odor, and does not dewater readily.

The characteristics of anaerobically digested sludge include: earthy, tarry odor, grainy, fiber-like texture, dark color, composed of 8–10% solids, 40–50% of the content is volatile, dries readily, no gray or green streaks, and has near-neutral pH.

There are two stages of digestion in all types of anaerobic digestion: the acid stage and the methane fermentation stage. The stages occur slowly at low temperature and faster as the temperature increases. In the acid stage, facultative bacteria called acid formers convert organic material into volatile (organic) acids. Vinegar is one of those acids. Three conditions exist in the acid stage: pH level is 5.0–7.0, there is little stabilization, and both hydrogen sulfide (H2S) and carbon dioxide (CO2) are produced. In the methane fermentation stage, anaerobic bacteria called methane formers convert the volatile acids into methane gas. Carbon dioxide (CO2) forms during all stages and should be 15–30% of the digester gases. In a well operated anaerobic digester, methane content is about 65–75%.

In the acid stage, facultative bacteria called acid formers convert organic material into volatile (organic) acids. Vinegar is one of those acids. Three conditions exist in the acid stage: pH level is 5.0–7.0, there is little stabilization, and both hydrogen sulfide (H2S) and carbon dioxide (CO2) are produced.

In the methane fermentation stage, anaerobic bacteria called methane formers convert the volatile acids into methane gas. Carbon dioxide (CO2) forms during all stages and should be 15–30% of the digester gases. In a well operated anaerobic digester, methane content is about 65–75%.

In a well operated anaerobic digester, methane content is about 65–75%. NOTE: this one passage is referenced four times in a row.... might be worth remembering....

Anaerobic digesters are covered to keep out free oxygen, to trap the useful methane gas, and to keep the digester warm. The tanks use floating covers or fixed covers. A floating cover is gastight and moves up or down with the sludge level or gas pressure. A fixed cover is dome shaped or a slab.

Mixers combine fresh sludge with bacterially seeded sludge. Mixing methods are gas, mechanical, or pump. Mixing is intermittent or continuous.

To maintain the desired temperature range, the sludge is pumped through pipes heated with hot water. Heating decreases digestion time from 30 days to 15 days.

Frequently load anaerobic digesters with small amounts of sludge. This can be accomplished using pumps (either centrifugal pumps or, more commonly, positive displacement pumps) due to the high solids content. A sludge content of 2–4% is recommended. >Overloading leads to foaming, poor gas production, low pH, and other problems.< Only digested sludge should be removed. Streaks of gray or green indicate that the sludge is not stabilized.

Frequently load anaerobic digesters with small amounts of sludge. This can be accomplished using pumps (either centrifugal pumps or, more commonly, positive displacement pumps) due to the high solids content. A sludge content of 2–4% is recommended. Overloading leads to foaming, poor gas production, low pH, and other problems. Only digested sludge should be removed. >Streaks of gray or green indicate that the sludge is not stabilized.

In normal operation, the contents of an anaerobic digester separate into layers. The layers are composed of gas, scum, supernatant liquor, digesting sludge, and digested sludge.

The methane can heat the digester or buildings, fuel engines, or heat water to produce steam. The excess gas can also be wasted through gas flares.

Anaerobic supernatant liquor is gray liquid that separates from the sludge. Supernatant liquor should be withdrawn when fresh sludge is added.

The following tests on raw and digested sludge help to operate anaerobic digesters: • pH • Alkalinity • Temperature • Volatile acids • Percent CO2 • Percent total and volatile solids

Anaerobic digester trouble signs include low pH, increased CO2, decreased alkalinity, and increased volatile acids.

When digester temperature is changed, a 1°C or less change per day prevents process upset.

Aerobic digestion stabilizes organic matter by aerobic means. Aerobic digestion is an endogenous process. Endogenous means it is an internal process. When there is a lack of food, the bacteria consume themselves. Aerobic digestion takes place in an activated sludge aeration tank, if the detention time is long enough. The TCEQ requires sludge to be in an aerobic digester 40 or 60 days, depending on temperature (30 TAC 217.249(t)(4)(B)). Extended aeration-activated sludge plants digest sludge in the aeration tank, but in other processes, aerobic digestion takes place in a separate aerated tank. Composting is a form of aerobic digestion done with dewatered sludge in open air instead of in an aerated tank. Heat generated by biological activity kills pathogens.

Methods of dewatering/thickening sludge to reduce its bulk include the following: • Drying beds • Belt presses • Rotary presses • Rotary vacuum filtration • Plate and frame presses • Centrifuges • Gravity thickeners (sedimentation tanks) • Dissolved air flotation

The disposal methods for digested and dewatered sludge include the following: • Process it • >Apply it to land for beneficial use< • Dispose of it in any other way, including >incineration< Class A, AB, and B sludges are classified based on Biosolids meeting the metal limits and the pathogen reduction requirements. Land application is the best use of properly treated sludge because this meets the EPA’s goal of beneficial reuse. >When sludge is disposed of by land-application it supplies nutrients to the soil. Sludge disposed in a landfill must have a solids content of at least 20%. Incineration is an expensive method of sludge disposal.

The disposal methods for digested and dewatered sludge include the following: • Process it • Apply it to land for beneficial use • Dispose of it in any other way, including incineration Class A, AB, and B sludges are classified based on Biosolids meeting the metal limits and the pathogen reduction requirements. Land application is the best use of properly treated sludge because this meets the EPA’s goal of beneficial reuse. When sludge is disposed of by land-application it supplies nutrients to the soil. >Sludge disposed in a landfill must have a solids content of at least 20%.< Incineration is an expensive method of sludge disposal.