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Anaerobic Digestion Overview
Purpose
Anaerobic digestion is a series of biological processes in which bacteria break down biodegradable material in the absence of oxygen. The process is widely used to treat wastewater solids because it reduces the feed solids volume and mass and produces digester gas containing approximately 65% methane that is usable as a fuel.
Theory of Operation
Anaerobic sludge digestion progresses in two steps. In the first step, facultative and anaerobic bacteria, called acid-forming bacteria, convert the complex organic material in the sludge to simple organic material, primarily organic acids. Some carbon dioxide is formed and some stabilization occurs during the first step. In the second step, the organic acids are converted to carbon dioxide and methane by anaerobic bacteria called methane-forming bacteria. Most of the sludge stabilization occurs in this step as the oxidizable organics are converted into gas, water, and a limited quantity of biological mass. Proper anaerobic sludge digestion is primarily based on food supply, time, temperature, pH, and mixing.

Sludge from the primary clarifiers (Digester 1 and 2 influent) should be pumped as continuously as possible over 24 hours. Organisms in the primary digester are most efficient when food (primary clarifier sludge) is furnished in small quantities at frequent intervals. If too much sludge is added to the primary digesters in a short period of time, the first (acid forming) step may become so predominant that the environment will become unfavorable (high volatile acids and low pH) for the organisms responsible for the second (gas forming) step, resulting in incomplete digestion with accompanying odors. An accurate daily record of sludge pumped to the digesters should be maintained.

Good sludge digestion can be defined as 50 percent volatile solids destruction. This can occur in about 20 days with proper primary digester feed rates, temperature, pH, and mixing. Effective mixing of incoming sludge with the contents of the digester is necessary to provide all working organisms with their essential food supply and to maintain a uniform temperature throughout the primary digesters. The gas mixing system should be checked frequently and maintained in good condition.

Anaerobic treatment can proceed quite well with pH levels from about 6.6 to 7.6; however, the optimum range is from 7.0 to 7.2. Beyond these limits, digestion efficiency decreases rapidly. Close monitoring of both pH and alkalinity concentration is required to ensure proper digester performance and timely correction of potential problems.

The sensitive balance maintained between acid forming and methane forming bacteria is subject to upset. This upset is evidenced by a marked change in pH or alkalinity. Correction of an unsatisfactory condition should begin immediately.
Equipment
The anaerobic digestion system includes the following components: 
  • Anaerobic Digesters 1, 2, and 3
  • Gas Mixing Compressors
  • Hot Water Boiler
  • Boiler Circulation Pump
  • Hot Water Circulation Pumps
  • Pressure/Vacuum Relief Valve and Flame Arrester
  • Foam Separators
  • Gas Pressure Transmitters
  • Waste Gas Burner
Description
Digestion Overview
Waste sludge streams from the primary clarifier are pumped into the center of the digester through a common line with the re-circulated contents of the primary digester. The circulated sludge passes through an external heat exchanger which is heated by hot water. The water in the boiler is heated by sewage gas. An auxiliary natural gas line is also provided as a secondary fuel.

The contents of the primary digesters are kept in a mixed condition by a gas-mixing system which utilizes sewage gas. As the mixed streams enter the digester, an equal volume of digested sludge is displaced into an overflow box from Digester 1 and flows by gravity to the Digester 2. Digester 2 is essentially a separation chamber in which the sludge solids are settled from the supernatant and then pumped to a blend tank and then pumped to the belt filter press or pumped to the sludge loading stand pipe.

A third anaerobic digester (Digester 3) was built to permit secondary sludge and scum to be digested separately from primary sludge and scum. Separate digestion of secondary sludge was desired to reduce the potential for foaming, which had been observed in the past during attempts to digest primary and secondary sludge together in the digesters.

Digester 1 and 2 were not designed for foaming. Digester foaming may still oc­cur even when the WAS is digested separately. Digester 3 is designed to pre­vent foam from entering the digester gas collection system and foam separators have been added at the other digesters.
WAS Digestion
Digester 3 was designed for single-stage, high-rate mesophilic digestion of thick­ened waste activated sludge (WAS) and secondary scum. Design Criteria for Digester 3. The digester was designed to be maintained at a temperature of about 95°F. This is accom­plished by circulating sludge through a heat exchanger located in the basement of the solids building. In the heat exchanger, heat is transferred to the sludge from a circulating hot water system. The heat exchangers and sludge circulation pumps are described in a later section on the hot water system.

Although an overflow has been provided, the design does not provide for the digester to be decanted. Little thickening of WAS was experienced in the digesters when WAS was digested, and decanting produced poor-quality supernatant. This ex­perience is typical when digesting WAS. Two sludge sampling ports in the roof of the digester have been provided; one ex­tends to elevation 637.5 (16.5 feet above the bottom of the sidewall), and the other to elevation 644.0 (23 feet above the bottom of the sidewall).

The minimum sidewater depth of 19.5 feet (EL 640.5) is the minimum needed to cover the sludge withdrawal port for circulating sludge to the heat exchanger. If the sludge level is drawn below this point, the digester cannot be heated. Liquid level in the digester is sensed with an ultrasonic sensor mounted in the center gas bonnet on the digester roof. Intended liquid levels for digesting sludge have been indicated above. This type of level sensor is likely to measure to the top of a dense scum layer, rather than to the liquid surface. It is likely to penetrate a light foam layer.

The digester will normally be fed WAS that has been thickened by the gravity belt thickener and conveyed from the thickener by the thickened sludge pump. The feed piping extends to the center of the digester at an elevation 6 feet above the bottom of the sidewall, and is also used to introduce heated recirculated sludge from the heat exchangers.
Manual adjustment of valves will permit primary sludge to be fed to Digester 3 or Di­gester 2, instead of Digester 1. This will permit cleaning of Digester 1.

Sludge is withdrawn from the center of the bottom of the digester through an 8‑inch line, either by gravity or using Sludge Transfer Pump 2. Sludge will normally be with­drawn to the sludge blend tank by gravity, from which it will be fed to the belt filter press for dewatering. When digested sludge is to be disposed of in liquid form, Sludge Transfer Pump 2 will be used to convey it to trucks. Sludge transfer is discussed in detail in a later section.

The digester overflow piping has been designed to give the operator the option of selecting one of two overflow elevations by opening or closing a knife gate valve. This will permit the retention of a large freeboard in the digester should severe foam­ing occur, or will permit an increase in the volume available for digestion should foaming not turn out to be so severe. Opening the knife gate valve will cause the di­gester to overflow at elevation 647.0 (26 feet above the bottom of the sidewall), and closing the knife gate valve will cause it to overflow at elevation 651.0 (30 feet above the bottom of the sidewall). A little less than 12,000 gallons of sludge added to or withdrawn from the digester will cause a 1‑foot change in liquid level.
Primary Sludge Digestion
Digester 1 is the main reactor in the two-stage anaerobic digestion process. The sludge streams previously described are introduced into an airless environment, heated, and mixed for approximately 20 days (mean detention time) before flowing into the 2 digester.

Digester 2 acts as a separation chamber which allows the digested sludge to settle and the clear supernatant to return to the plant flow. Some anaerobic decomposition occurs in the secondary digester. An Eimco supernatant selector has been provided to enable the operator to withdraw supernatant from varying levels within the secondary digester. The selector is operated by turning the crank operator until the indicator points to the desired level. This causes the supernatant to flow via the overflow box into the drain. An emergency overflow is provided to prevent overfilling the digester.
Digester  Mixing
Digester gas mixing Compressor 1 (GC‑8321) is dedicated to Digester 1. Compressor 2 (GC‑8322) is dedicated to Digester 3.  When both Digester 1 and Digester 3 are in use, both compressors are intended to operate continuously.

The discharge piping on the compressors is interconnected, and in the event that one compressor is out of service, manual adjustment of valves will permit the other to be used to mix either digester.  During normal operation, the valve on the interconnection should be closed, to provide equal gas flow to each of the two digesters.

The digester mixing system:
  • Promotes contact between the feed solids and the anaerobic microorganisms
  • Distributes alkalinity throughout the tank, to prevent pH depression
  • Minimizes digester volume reduction caused by scum/foam accumulation
  • Minimizes digester temperature gradients
  • Dilutes the inhibitory byproducts of microbiological reaction throughout the sludge mass
Digested Solids Transfer
Digested sludge is normally removed from Digester 2 using Sludge Transfer Pump 1, and is pumped to the sludge blend tank for blending prior to de­watering. Digester 1 overflows into Digester 2, and sludge is not normally pumped from Digester 1. Sludge Transfer Pump 1 can, however, be used to remove sludge from Digester 1 if necessary.

Because the sludge level in the digesters is normally much greater than that in the blend tank, an automatic valve has been provided on the discharge of each sludge transfer pump to prevent any flow through the pump when it is off. This valve will automatically open when the pump comes on and close when the pump shuts off.

Digested sludge from Digester 3 is normally removed by gravity and routed to the sludge blend tank for dewatering. Pumping is not necessary under normal operat­ing conditions because of a favorable difference in head between Digester 3 and the sludge blend tank. An automatic control system has been provided for transfer­ring sludge from Digester 3 to the sludge blend tank by gravity at a con­trolled flow rate that can be adjusted by the operator.
Liquid Solids Loading
Liquid digested sludge can be loaded into trucks either directly from the digesters, from the sludge blend tank or after thickening by the GBT. The liquid sludge loading sta­tion consists of a sludge loading arm that hangs from the ceiling of the truck load­ing area and pivots around two swivel joints, and a pendant mounted pump con­trol station.

When digested sludge is to be thickened prior to loading in liquid form, it will first be transferred to the sludge blend tank, from which it will be fed to the GBT by the sludge feed/load pump.

All of the options above require manual adjustment of several valves to route the di­gested sludge to the sludge loading arm, diverting it from its normal pathway, which is to the belt filter press (BFP) for dewatering.  The operator then transfers manual control of the pumps to be used for truck loading to the pendant control station in the truck loading area.  
Digested Sludge Blending
Sludge will normally be removed from Digester 2 and Digester 3, mixed to­gether in the sludge blend tank, and dewatered by the BFP. The sludge blend tank permits the blending of digested primary sludge from Digester 2 with digested thickened WAS from Digester 3 prior to dewatering. It has been observed that more effective dewatering of digested WAS can be achieved when the sludges are blended rather than dewatered separately. The blend tank can also be used for feed­ing digested sludge to the GBT for thickening.

The sludge blend tank has a capacity of approximately 12,500 gallons. The head­space in the sludge blend tank is interconnected with the digester gas distribution system (described in a later section). The tank should be kept sealed, and all of the precau­tions taken when opening and closing the covers on the digesters should be taken during maintenance operations on the tank.

Sludge is withdrawn from the bottom of the tank by the sludge feed/load pump. The tank will overflow to the head of the plant. The overflow has a trap and a trap primer valve to prevent the escape of digester gas to the drain system. 
Recirculation and Heating
The heat source for the hot water system is a single boiler.  Heat sinks for the hot water system are the various com­ponents of the space heating systems, two spiral heat exchangers for heating sludge, and a plate and frame Auxiliary Heat Exchanger for heating polymer make-up water.

Hot water from the boiler is circulated by the Boiler Circulation Pump to and from a primary hot water loop. Hot water is circulated through the primary loop by two Primary Hot Water Circulation Pumps. Hot water is withdrawn from and returned to this primary loop by three secondary loops, which have their own circulation pumps.

The first secondary loop is for space heating. The pumps for this secondary loop circulate the hot water to the various end use space heating devices in the Solids Building and in the Headworks. The second secondary loop serves sludge Heat Exchanger 1.  Hot water is circu­lated to and from the primary hot water loop and through the sludge heat exchanger by Heat Exchanger Circulation Pump 1.  Sludge Circulation Pump 1 withdraws sludge from Digester 1, pumps it through the cold side of the heat exchanger, and returns it to the digester.

The third secondary loop serves sludge Heat Exchanger 2.  Hot water is circu­lated to and from the primary hot water loop and through the sludge heat exchanger by Heat Exchanger Circulation Pump 2.  Sludge Circulation Pump 2 with­draws sludge from Digester 3, pumps it through the cold side of the heat exchanger, and returns it to the digester.

Hot water circulating in the primary hot water loop directly flows through the Aux­iliary Heat Exchanger. Plant water (W1) flows through the cold side of this heat exchanger and then serves as polymer make up water at the four polymer tanks.
Sludge Feed/Load Pump and Grinder
The Sludge Feed/Load Pump normally feeds sludge from the Sludge Blend Tank to the Belt Filter Press (BFP) for dewatering. It can also be used to feed sludge from the Sludge Blend Tank to the Gravity Belt Thickener (GBT). This pump can also be used to load liquid sludge from the Sludge Blend Tank in to trucks.

The grinder reduces the size of large solid materials that have accumulated in the di­gested sludge after the sludge leaves the Sludge Blend Tank and before it enters the Sludge Feed/Load Pump. The purpose is to reduce the potential of large solids punc­turing the belts during dewatering.
Digester Gas Utilization System
Digester gas is used to mix and heat the digesters and to provide space heating in the winter for the Solids Building and the Headworks Building. Digester 2 is equipped with a floating cover that was designed to maintain a constant pres­sure in the system of 6 inches water column. Digester gas is collected in the center bonnets of the three digesters and piped together into a single distribution system that is also interconnected to the headspace in the sludge blend tank. Sedi­ment traps and foam separators have been provided on the piping from all three di­gesters. Each digester has a pressure relief system.
Anaerobic Digesters6
Two fixed cover anaerobic digesters and one floating cover digester provide the volume required for the digestion process. Digested gas is collected from all three digesters. 
Digester Mixing Gas Compressors6
  Digester gas is piped from the common distribution system to two 70 scfm sliding vane compressors, which discharge back to diffusers in the bottom of Digesters 1 and 3 for mixing.

The gas mixing system was supplied as a package, and includes gas filters, two sliding vane compressors, oil separators, pressure switches, pressure relief valves, motorized unloading valves, flow meters, control panels, gas distribution balancing devices and the diffusers within the digesters. Literature provided by the package system supplier should be consulted for operation of the system.
Hot Water Boiler6
The heat source for the hot water system is a single boiler with a nominal 113 hp, 3.8 million Btu per hour output capacity. 
Heat sinks for the hot water system are the various com­ponents of the space heating systems, two spiral heat exchangers for heating sludge, and a plate and frame Auxiliary Heat Exchanger for heating polymer make-up water.
Boiler Circulation Pump 6
Hot water from the boiler is circulated by the Boiler Circulation Pump to and from a primary hot water loop.
Hot Water Circulation Pumps6
Hot water is circulated through the primary loop by two (2 )Primary Hot Water Circulation Pumps. Hot water is withdrawn from and returned to this primary loop by three secondary loops, which have their own circulation pumps.
Flame Arrester 6
A flame arrester is provided on the gas line prior to termination at the hot water boiler. The flame arrester works by forcing the burning gas through a maze of close fitting plates thereby separating the flame and dissipating the heat.

The maximum pressure drop through a flame arrester is 1 psig with a maximum working pressure of 10 psig.
Digester Pressure/Vacuum Relief Valve6
The pressure relief/vacuum breaker valve and flame arrester assembly has three functions:
  1. It is used to prevent gases within the digester from building up in excess of the specified pressure, by allowing excess gases to pass from the digester to the atmosphere.
  2. It also allows air to pass through into the digester to replace gas or sludge if either is drawn from the unit too rapidly. Therefore, sludge and/or gas should be drawn slowly from the digester to prevent the vacuum within the unit from falling below atmospheric pressure.
  3. The flame arrester does not allow flames to enter the digester causing an explosion.
Digester Gas Pressure Transmitters6
The digester gas pressure sensors transmit a pressure signal to the PLC. The PLC utilizes this signal to control the gas pressure in the system ensuring sufficient pressure is available to operate the hot water boiler while wasting any excess gas to the waste burner when the pressure starts to climb. 
Foam Separators 6
Foam separators route the digester gas flowing through them under a water spray fed by the W1 system. The water piping is provided with valves for adjusting flow rate and balancing distribution to three spray nozzles. The drain line from each foam separa­tor is fitted with a trap and a valve for isolating the digester gas from the drain system.
Waste Gas Burner6
Excess gas is wasted and burned at the waste gas burner. The waste gas burner is designed with an automatic lighting system which uses LP gas to maintain a continuous pilot flame.

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Last Updated: 9/16/2013 9:49:37 AM
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