6. Sludge Treatment and Disposal

Wastewater Engineering

refers to concentrating impurities into solid form and then separating these solids from the bulk liquid.

Sludge

the end products of wastewater sludge treatment processes.

Biosolids

treated or stabilized sludge

Biosolids

are the organic products of primary and secondary wastewater treatment processes that can be beneficially reused

Biosolids

main goal of sludge treatment

to convert it to an inert solid product that can be safely disposed in the environment or reused for useful purposes

the goal of biosolids/sludge treatment before sludge disposal

Turning “sludge” to “biosolids”

Include all types of organic and inorganic materials large enough to be removed on bar racks.

Screenings

Heavier inorganic solids that settle w/ relatively high velocities.

Grit

Floatable materials skimmed from the surface of primary & secondary settling tanks and from grit chambers and chlorine contact tanks

Scum/grease

Sludge from primary settling tanks; usually gray and slimy and has an extremely offensive odor. Primary sludge can be readily digested under suitable conditions of operation.

Primary Sludge

▪ Has a brown flocculent appearance. If the color is dark, the sludge may be approaching a septic condition.

▪ If the color is lighter than usual,there may have been underaeration w/ a tendency for the solids to settle down.

▪ Sludge in good condition: Has an inoffensive “earthy” odor.

▪ Activated sludge will digest readily alone or when mixed w/ primary sludge.

Activated sludge

▪ Humus sludge, brownish, flocculent, and relatively inoffensive when fresh.

▪ Digests readily

Trickling filter sludge

▪ Are brown to dark brown and have flocculent appearance.

▪ The odor is not offensive; it is characterized as musty.

▪ Well digested aerobic sludge dewater easily on drying beds.

Aerobically digested biosolids

▪ Are dark brown to black and contain an exceptionally large quantity of gas.

▪ When thoroughly digested, they are not offensive, the odor being similar to burnt rubber or sealing wax.

▪ Primary sludge, when anaerobically digested, produces about twice as much as CH4 gas as WAS does.

Anaerobically digested biosolids

▪ Sludge from chemical precipitation w/ metal salts is usually dark in color. ▪ May be red if it contains much iron.

▪ Lime sludge is grayish-brown.

▪ Odor may be objectionable but not as offensive as the odor of primary sludge.

▪ Chemical sludge is somewhat slimy, the hydrate of iron or aluminum makes it gelatinous.

▪ If the sludge is left in the tank, it undergoes decomposition similar to primary sludge, but at a lower rate.

Sludge from chemical precipitation

▪ Depends on the unit process.

▪ Chemical sludge from phosphorus removal is difficult to handle and treat.

▪ Tertiary sludge combined with biological nitrification and denitrification is similar to WAS.

Sludge from tertiary (advanced) treatment process

Organic solids, grit inorganic fines, greasy, odorous slurry. Includes: 1% tank skimmings; 4-6% solids concentration and 60-80% VSS.

Primary Sludge

Active Microbial mass, dark brown suspension, VSS: 70-80%

Waste Activated Sludge

Similar to activated sludge

Trickling Filter Sludge (Humus)

Title 40 of the Code of Federal Regulations (CFR)

“The Standards for the Use and Disposal of Sewage Sludge”(US EPA,1993)

reduce pathogens below detectable levels and there are no pathogen-related restrictions for land application. (Class A Biosolids)

PFRP – Processes to Further Reduce Pathogens

reduce but do not eliminate pathogens. (Class B Biosolids)

PSRP – Processes to Significantly Reduce Pathogens

reduce the pathogens in biosolids (fecal coliform, FC, or Salmonella sp. bacteria, enteric viruses, parasites, and viable helminth ova) to below detectable levels.

CLASS A BIOSOLIDS

ensure that pathogens have been reduced to levels that are unlikely to pose a threat to public health and the environment under specific conditions of use

CLASS B BIOSOLIDS

using either within vessel os static aerated pile, temperature of biosolids at 55 oC for 3d

PFRP: Composting

using either within vessel os static aerated pile, temperature of biosolids at >40 oC for 5d

PSRP: Composting

dewatered biosolids are dried by direct or indirect contact w/ hot gases to reduce H2O content to 10%

PFRP: Heat Drying

biosolids are dried on sand beds or on paved or unpaved basins minimum of 3 months.

PSRP: Air drying

liquid biosolids are agitated w/ air or no oxygen to maintain aerobic conditions, and the MCRT is 10d at 55 -60 oC

PFRP: Thermophilic Aerobic Digestion

biosolids are irradiated w/ beta rays from an accelerator at dosages of at least 1 megarad

PFRP: Beta-ray Irradiation

temperature of biosolids is maintained at > 70oC for 30 minutes

PFRP: Pasteurization

biosolids are agitated w/ air or no oxygen to maintain aerobic conditions, for an SRT and temperature between 40d at 55oC and 60d at 60 oC

PSRP: Aerobic Digestion

biosolids are treated between an SRT of 15d at temperature of 35 to 55oC and an SRT of 60d @ 60oC

PSRP: Anaerobic Digestion

sufficient lime is added to raise the pH of the biosolids to pH 12 and is maintained for 2 hrs of contact.

PSRP: Lime Stabilization

procedure used to increase the solids content of sludge by removing a portion of the liquid fraction or separating as much water as possible by gravity or flotation.

Sludge Thickening

Thickening is achieved by the ff methods

a. Gravity thickening

b. Flotation thickening

c. Mechanical thickening

Mechanical Thickening

a. Centrifuges

b. Rotary drums

c. Filter belts

uses gravity forces to separate solids from the sludge.

Gravity Thickening

performed in circular settling tanks that are equipped with scraper arms having vertical pickets, w/c serve to gently agitate the sludge and aid in the release of trapped gases and water.

Gravity Thickening

when primary clarifiers are used to thicken solid for downstream processing. To thicken sludge, a sludge blanket must be created to consolidate solids w/o allowing the clarified water to be pulled through.

Co-settling Thickening

is achieved by releasing fine air bubbles that attach to sludge particles and cause them to float.

Flotation Thickener

used for wastewater sludge thickening in the USA.

dissolved air flotation (DAF)

factors that must be considered in the design of DAF system:

Air-to-solid ratio

Hydraulic loading

Polymer addition

Solids-loading rate

computed as the mass ratio of air available for flotation to the solids to be floated in the feed system. Adequate flotation is achieved in most municipal wastewater thickening applications at ratios of 0.02:1 to 0.06:1… single most important factor affecting DAF performance.

Air-to-solid ratio

the rate of hydraulic loading is calculated as the sum of the feed and recycle flow rates divided by the net available flotation area. Typical designs are in the range of 30 to 120 m3 /d ・ m2 of surface area.

Hydraulic loading

Typical polymer dosage is in the range of 2 to 5 g dry polymer/kg of dry feed solids. Bench scale testing is the best method to determine the optimum chemical conditioning polymer and dosage.

Polymer addition

calculated as the mass of solids per hour per effective flotation area.

Solids-loading rate

may be used as a thickening device for activated sludge or as a dewatering process for digested or chemically conditioned sludge.

Centrifuge

• most widely used

• rotates along a horizontal axis and operates in a continuous-feed manner.

• consists of a rotating bowl having a cylindrical-conical shape and a screw conveyer

Solid Bowl Centrifuge

• called the imperforate bowl, is a knife-discharge type and operates on a batch basis.

• particularly well-suited for soft or fine solids that are difficult to filter or where the nature of the solids varies widely.

• solids fall through the bottom of the bowl into a hopper. The plow retracts and the bowl accelerates, starting a new cycle.

Basket Centrifuge

• rotates along a vertical axis and operates in a continuous manner.

• The centrifugal force is applied to the relatively thin film of liquid and solid between the discs.

Disc-nozzle Centrifuge

development of this device stemmed from the application of belt presses for sludge dewatering.

Gravity Belt thickener

• function similarly to the gravity belt, where free water from a flocculated sludge drains through a rotating cylindrical stainless steel screen.

• can be used as a prethickening step before belt-press dewatering and are typically used in small-to medium plants for WAS thickening.

Rotary drum thickeners

Is among the oldest processes used for the stabilization of solids and biosolids especially used for high organic load.

Anaerobic Digestion

Three (3) types of chemical and biochemical reactions that occur in anaerobic digestion

(1) hydrolysis;

(2) fermentation; and

(3) methanogenesis.

“solubilization and size reduction” of organic matter by extracellular enzymes produced by bacteria.

Hydrolysis

the addition of water (“hydro”) to complex molecules by bacteria to split (“lysis”) unique chemical bonds in the complex molecules, thereby permitting the production and release of simple and soluble molecules.

Hydrolysis

enzymes that breakdown celluloses (the structural component of primary cell wall) produced by bacteria, fungi, and protozoa.

Cellulases

enzymes that breakdown of starch into sugars (present in human saliva produced by pancreas).

Amylases

enzymes that breakdown of protein bonds into amino acids

Proteases

two important groups of fermentative bacteria:

the acidogenic bacteria and

the acetogenic bacteria

Conversion of simple sugars, amino acids, and fatty acids to (1) organic acids such as acetate, butyrate, formate, lactate, and succinate, (2) alcohols such as ethanol and methanol, (3) acetone, and (4) carbon dioxide, hydrogen, and water.

Acidogenesis (Acid production process)

conversion of organic acids and alcohol to acetic acid (CH3COOH) (by acetogens)

Acetogenesis

Carried out by the organisms known collectively as methanogens (methane formers).

Methanogenesis

2 Groups of Methanogens:

(1) Aceticlastic (acetoclastic) methanogens

(2) Hydrogen-utilizing methanogens (hydrogenotropic methanogens)

– Produce methane by splitting acetate

– split acetate into CH4 and CO2.

– Methanosarcina & Methanothrix (also termed as Methanosaeta) are the only organisms to use acetate to produce CH4 and CO2

Aceticlastic (acetoclastic) methanogens

use H2 as the electron donor and CO2 as the electron acceptor to produce CH4

Hydrogen-utilizing methanogens (hydrogenotropic methanogens)

Acetate

CH3COOH

Carbon dioxide

CO2

Carbon monoxide

CO

Formate

HCOOH

Methanol

CH3OH

Methylamine

CH3NH2

most important factors affecting the performance of anaerobic digesters

• solids residence time

• hydraulic residence time

• temperature

• pH

• toxic materials

Anaerobic digesters are usually heated to maintain a temperature of __________

34—36˚0C (94-970˚F)

Anaerobic Digester usual pH level

6.8 to 7.2

Two types of Anaerobic Digestion

1. Standard (Low) Rate Process

2. High Rate Process

does not employ sludge mixing, but rather the digester contents are allowed to stratify into zones, SRT (detention time): 30 to 60 days

Standard (Low) Rate Process

w/ supplemental mixing and heating, SRT: 15 to 20 days and two digesters operating in series separate the functions of fermentation and solids-liquid separation and residual gas extraction.

High Rate Process

Primary Digester

reactor is covered, heated, and mixed to increase the reaction rate. The temperature of the sludge is usually about 35°C.

not mixed or heated and is used for storage of gas and for concentrating the sludge by settling

Secondary digester

looks similar to an upright egg and is different from the conventional American digester and the conventional German digester

Egg-shaped digester

used to stabilize primary sludge, secondary sludge, or a combination of these by long-term aeration. The process converts organic sludge solids to carbon dioxide, ammonia, and water by aerobic bacteria with reduction of volatile solids, pathogens, and offensive odor

Aerobic Digestion

Aerobic Digestion with a concentration of influent VSS must be no more than 3 % for retention times of 15 to 20 days.

Conventional Aerobic Digestion

Aerobic Digestion with a variation of both conventional and high purity oxygen aerobic digestion, and the process requires insulated reactors (120 – 140F), aerators, foam controllers and sludge.

AutoThermal Thermophilic Aerobic Digestion (ATAD)

This aerobic digestion is used in lieu of air, particularly applicable in cold weather climates because of its relative insensitivity to changes in ambient air temperature due to the increased rate of biological activity.

High-Purity Oxygen Aerobic Digestion

Aerobic Digestion with low temperature ( less then 20o C).

Cryophilic Aerobic Digestion

Advantages of Aerobic sludge digestion

• Volatile solids reduction in a well-operated aerobic digester is approximately equal.

• Lower level of BOD and phosphorus in the supernatant.

• Production of an odorless, humus like, biologically stable end product.

• Greater sludge fertilizer value

• Simplicity in operation and maintenance.

• Lower capital cost

Disadvantages of Aerobic sludge digestion

• High power cost is associated w/ supplying the required oxygen.

• Digested biosolids produced have poorer mechanical dewatering characteristics.

• Process is significantly affected by temperature, location, tank geometry, concentration of feed solids, type of mixing device and tank material.

a process in which organic material undergoes biological degradable to a stable end product.

Composting

Optimum moisture content for composting

50 – 60% water

an organic material added to the feed substrate, primarily to reduce the bulk weight and increase the air voids for proper aeration.

Amendment (ex: sawdust, straw, recycled compost, rice hulls)

an organic or inorganic material used to provide structural support and to increase the porosity of the mixture for effective aeration

Bulking agent (ex: wood chips)

Two Principal Methods of Composting

1. Agitated Composting

A. Windrow Composting

B. In Vessel Composting

2. Static Composting

A. Static Aerated Pile Composting

A unit operation used in the past for conditioning sludge.

Elutriation

used for determination of specific resistance of sludge. It is a method of testing sludge drainability or dewatering characteristics using various conditioning agents

Buchner Funnel test

relies on gravity and the capillary suction of a piece of thick filter paper to draw out water from a small sample of conditioned sludge

Capillary Suction Time Test (CST)

easiest method to use, testing standard volumes of sludges with different conditioner concentrations followed by rapid mixing, flocculation, and settling

Standard Jar Test

a physical unit operation used to reduce the moisture content of sludge and biosolids by subjecting it to vacuum, pressure or drying

Sludge Dewatering

the dewatered solids coming out from a dewatering device

Cake