Chapter 5-Sludge Processing and Land Application Flashcards
The main purpose of sludge processing is to?
- Reduce the water and/or organic content of the sludge.
- Render the sludge suitable for final disposal or reuse.
-Thickening, conditioning, dewatering and drying are all principally used to remove moisture
from the sludge.
- Digestion, composting and incineration are used primarily to stabilize the organic matter in
the sludge.
Draw the two typical solid process flow diagrams
What is Slude dewatering used for in a process plant?
Sludge dewatering is used to remove as much water as possible from a sludge to produce a highly concentrated cake. The capital and operating expenses of these devices are usually significantly less than the costs of hauling large quantities of sludge long distances to landfill or other disposal site.
How is slude dewatering done?
- Belt filter presses- are used for thickening and dewatering – the press operates on the principle that bending a sludge cake contained between two filter belts.
- Centrifuge- In this centrifuge, slurry is input into a central feed pipe. The bowl is rotated typically between 200 and 8000 rpm, creating the centrifugal force that moves the particles to the wall of the bowl.
- Gravity Thickener- Gravity thickening is one of the most common methods used and is accomplished in a tank similar in design to a conventional sedimentation tank. The feed sludge is allowed to settle and compact, and the thickened sludge is withdrawn from the conical tank bottom.
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Flotation Thickener (e.g. Dissolved Air Flotation (DAF)) - Air is dissolved in water at pressures above atmospheric pressure in enclosed units. This water, saturated in
air with respect to atmospheric pressure, is then fed into the vessel with wastewater at atmospheric
pressure.
How does a Floatation Thickener work?
Air is dissolved in water at pressures above atmospheric pressure in enclosed units. This water, saturated in air with respect to atmospheric pressure, is then fed into the vessel with wastewater at atmospheric pressure. Bubble formation occurs at the pressure reducing valve and dissolved air is released from solution in the flotation vessel. Bubbles are formed which are entrapped in flocs or adhere to flocs and buoy them to the surface. A skimmer collects the floating solids and a portion of the effluent is recycled through the air dissolution tank.
What is Stabilization?
• Sludges are stabilized to (1) reduce pathogens, (2) eliminate odors, and (3) reduce, inhibit or eliminate the potential for putrefaction.
- Survival of pathogens, release of odors, and putrefaction occur when microorganisms are allowed to flourish in the organic fraction of the sludge.
- The means to eliminate these nuisance conditions through stabilization is mainly related to the biological reduction of the volatile content and the addition of chemicals to the solids or biosolids to render them unsuitable for the survival of the microorganisms
- The principal methods used for stabilization are (1) alkaline stabilization, (2) anaerobic digestion, (3) aerobic digestion and (4) composting.
Describe Akaline Stabilization with lime
- Quick lime and hydrated lime is most commonly used. Quicklime exothermic reaction raise the temperature of the mixture above 50 degees (inactive worm eggs). Its objective is to maintain the pH above 12 for about 2 hours to provide enough residual alkalinity so the pH does not drop below 11 for 7 days.
What are the advantages and disadvantages to Quicklime?
Describe Anarobic Digestion and its key components
Anaerobic digestion involves the decomposition of organic and inorganic matter in the absence of molecular oxygen. Anaerobic digestion of municipal wastewater sludge can, in many cases, produce sufficient digester gas to meet most of the energy needs for plant operation.
Key Components (1) A closed vessel often made of concrete and steel. (Often insulated to prevent Heat Loss) (2) A mixing system to promote uniform bioreactor conditions.(Gas recirculation or mechanical mixing) (3) A heating system. (4) A gas-liquid-solids separation system.
What is Aerobic Digestion
- Thermophilic aerobic digestion processes, which have higher solids stabilization and pathogen inactivation rates, have been developed allowing aerobic digestion to remain a viable option for the stabilization of waste solids.
- During aerobic digestion, particulate organic matter is hydrolyzed and converted into biodegradable soluble organic matter. The biodegradable soluble organic matter is then converted into carbon dioxide, water, and active biomass.
What is Aerobic Digestion
- Thermophilic aerobic digestion processes, which have higher solids stabilization and pathogen inactivation rates, have been developed allowing aerobic digestion to remain a viable option for the stabilization of waste solids.
- During aerobic digestion, particulate organic matter is hydrolyzed and converted into biodegradable soluble organic matter. The biodegradable soluble organic matter is then converted into carbon dioxide, water, and active biomass.
What is Composting?
Composting is a process in which organic material undergoes biological degradation to a stable end product. Sludge which has been composted properly is a nuisance-free humus-like material. Approximately 20 – 30% of the volatile solids are converted to carbon dioxide and water.
Properly composted biosolids may be used as soil conditioners in agricultural or horticultural applications, subject to any limitations based on the constituents in the composed biosolids.
What are the basic steps to composting?
- Preprocessing – the mixing of dewatered sludge with an amendment and/or bulking agent. An amendment is an organic material (e.g. straw or recycled compost) used to reduce bulk weight, reduce moisture content, and increase air voids). A bulking agent is an organic or inorganic material (e.g. wood chips) used to provide structural support and to increase porosity of the mixture for effective aeration.
- High-rate decomposition, aerating the compost pile either by the addition of air, mechanical turning, or by both. Temperatures in the range of 50 to 70oC can be reached during this phase.
- Recovery of the bulking agent.
- Curing and storage stage – further stabilization and curing of the compost.
- Post-processing – screening for the removal of nonbiodegradable material such as metals and plastics or grinding for size reduction.
What are the two most common types of composting?
- Aerated Static Pile
- Bulking agent is typically wood chips which is mixed with dewatered sludge by a rotating drum or a front end loader. Material is composted for 20 to 28 days and is typically followed by a curing period of 30 days or longer. Plastic drainage pipe underneath the compost is used for aeration. The piles are typically 2 to 2.5 m high often covered with an insulation layer and enclosed for process and odor control. - Windrow
-Windrows are constructed from 1 to 2 m high and 2 to 4.5 m at the base. The rows are turned and mixed periodically during the composting period. The composting period is about 21 to 28 days.
Temperatures are maintained at or above 55oC.in windrow composting, aerobic conditions are difficult to maintain therefore microbial activity may be
aerobic or anaerobic or various combinations thereof. Specialized equipment may be used to turn the windrows, sometimes releasing offensive odors. Some windrow operations are covered or enclosed, similar to aerated static piles.
