Anaerobic Digestion And Co-Digestion Optimization

ByTodd Williams, PE, BCEEResiduals Resource Recovery Global Technology LeaderAndTim Shea, PE, PhD, BCEEOhio Water Environment Association Biosolids ConferenceColumbus, OHDecember 5, 2013Copyright 2013 by CH2M HILL, Inc. alWBG062211121245ORLAnaerobic Digestion and Co-Digestion Optimization

OutlineBasics Mixing Preventing Digester Overflows Addition of FOG and HSW Case Studies 2Copyright 2013 by CH2M HILL, Inc.

Digester Basics The operator’s work begins after the design and construction arecompleted. This discussion assumes that these steps were donecorrectly – not always the case.The basic information needed for digester start-up and operationis found in the Manuals of Practices of WEF (MOPs) for anaerobicdigestion, starting with MOP 16, “Anaerobic Digestion Manual ofPractice”, published in the late 1980s.Co-digestion is an emerging practice, and there is no MOPavailable – yet – just experience.However there are the lessons learned at a number of facilitiesthat can be applied. This is our focus today.3Copyright 2013 by CH2M HILL, Inc.

Co-Digester Basics Co-Digestion programs have evolved from the realization thatsome wastes are better introduced directly to the digester ratherthan via the sewer into the headworks.Such wastes can be piped directly to digestion, or more commonlyas hauled wastes received via tankage for pre-treatment, blendingand pre-heating with sludge as digester feedstock.The largest such programs are found in the mid-west USA atplaces such as Des Moines, IA, in operation over 20 years.Many other utilities have ventured into FOG/HSW waste handlingincluding Johnson County, KS; Gwinnett County, GA; East BayMUD, CA; Fresno, CA; Davenport, IA; Hershey, PAThe waste receiving and handling practices used at these facilitiesprovide many lessons learned for newer facilities.4Copyright 2013 by CH2M HILL, Inc.

Co-Digestion Lessons Learned: Knowledge of the individual wastes to be accepted – what are awaste’s characteristics, variabilities and biomethane productionpotential.Contractual basis for acceptance – have a specification todescribe the basis for acceptance or rejection of a load.Assay procedures – have a standard procedure to ensure thateach load received is within specification.Outlet for the biogas – have established uses in place for theadded biogas production.Receiving storage capacity – it is always cheaper to storage wastethan to store gas.Additional gas hold-up in co-digestion will likely occur, reducingthe density of the digesting liquid and increasing volume.5Copyright 2013 by CH2M HILL, Inc.

Most Existing Digestion Systems Are Not Designedfor Co-Digestion: The scale of a co-digestion program will change over time as theviability and reliability of a program is built.Multiple categories of wastes to be received may require multiplewaste storage tanks.Waste receiving, blending and pre-heating operations must bescaled to the size of the program, often a guess, so phasing isoften a consideration.Digester design should but often doesn’t provide additional headspace and overflow capacity for when the unexpected happens.Secondary digester capacity is useful/essential for balancing flowsto dewatering.Other examples are provided later in this presentation.6Copyright 2013 by CH2M HILL, Inc.

Copyright 2013 by CH2M HILL, Inc. alWBG062211121245ORLDigester Mixing

Digester Mixing Concepts Too little mixing can allow pockets of gas to accumulate, creatingdensity gradientsToo much mixing can entrain gas on a wider scale, creatingdensity gradientsTemperature gradients and variability in feeding, increasingdensity gradientsRapid rise foam formation results from excessive and rapidlowering of density8

Digester Mixing Concepts (continued ) There will be some foaming in a digester under the best ofcircumstancesCo-digestates like FOG and food wastes add to the potential fornuisance foamingDigesters should be designed to accommodate some nuisancefoamingIncreasing mixing energy input has been a tempting panacea fordigester designers9

Hydraulic Mixing SystemsPump Mixing or Nozzle (Jet-Mix) MixingAdvantages of Hydraulic Mixing Provides sufficient mixing energy (eliminates dead spots) Maintains solids in suspension and re-suspension Chopper pumps macerate rags and debris accumulation (VaughnChopper Pump) Controls foam problems more effectively than gas mixing systems Reduces routine cleaning (minimal solids accumulation on digesterbottom) Easy retrofit of existing digester tanks (equipment located outside ofdigester) Special ventilation or electrical requirements not required Rotatable nozzles (adjust according to scour locations, Jet-Mix only) System requires least amount of submerged equipment (except forsubmersible mixers) Low explosive hazard during system maintenance (compared to gasmixing)10

Hydraulic Mixing SystemsPump Mixing or Nozzle (Jet-Mix) MixingDisadvantages of Hydraulic Mixing Limited history in digesters; excellent history in mixing sludgetanks Nozzle cranks penetrating walls of digester (Jet-Mix only) Slightly higher energy usage than confined gas systems11

Mechanical Mixing Systems Draft Tube Mixers (Eimco and WesTech)Center Mixers (Lightning/Philadelphia)Peripheral Mixers (Omnivore)Linear Motion Mixers (Ovivo)12

Mechanical Mixing SystemsAdvantages of Mechanical Mixing System provides a less explosive-hazard environment compared togas mixing Provides sufficient mixing energy for various tank sizes andconfigurations VFD pumps can alternate speed according to digester solids contentsDisadvantages of Mechanical Mixing More prone to clogging with rags and other large debris Formation of rag balls clog downstream pumps and piping Sensitive to liquid level in tank (mixing not as effective) Replacement of digester covers required to retrofit existing system(center mixer system) Reinforcement of digester cover required to handle heavy weight andforces generated by mixer (center mixer and roof-mounted draft tubes)13

Gas Mixing Systems Bubble Gun System (IDI Atara)Perth (Envirex)Draft Tube (Walker)CRP System (Chicago Pump Co.)14

Gas Mixing SystemsAdvantages of Gas Mixing Most commonly used mixing system Mixing intensity regulated by throttling gas flow Complete mixing possible with unconfined gas mixing systems, butonly IDI Atara will guarantee performance Potentially less power consumption than mechanical mixing systemsDisadvantages of Gas Mixing Gas compressors, nozzles, and diffusers prone to plugging Excessive foaming and grit accumulation experienced Explosive characteristics of digester gas (O&M more difficult) Costly retrofit for existing digesters (addition of gas compressors andpiping) Inefficient mixing below gas injection level prevents grit from remainingin suspension System requires maintenance of “different” types of equipment –compressors, etc. Proprietary mixing systems15

Mixing Parameters in Digesters GER (Gas Evolution Rate) – is driven by microbiological activity:–– RRFR (Rapid Rise Foam Formation Rate) is driven by–– Area-specific as gas always rises (Mixing Energy/area-time)Increases with greater height to diameter ratioGER (Gas Evolution Rate)Mechanical MEI (Mixing Energy input)Each sludge and digester design is a unique combination andpresent a unique set of circumstances.16

Where’s the Mixing Energy Sweet Spot?GERRRFROver-Mixing ZoneHere?Mixing Energy InputRRFR Rapid Rise Foam Formation RateGER Gas Evolution Rate17

Key Points on Digester Mixing Gas evolution rate increases in proportion to mixing energy input Digester over-mixing is a wide-spread concern and contributes tofoam production. More work and more detailed information required to selectappropriate digester mixing systems. Better performance results from feeding digesters as continuous aspossible, especially with FOG and high-strength organic wastesadded to the feed. Can expect to see– More pumped hydraulic jet mixing systems with VFDs (variablefrequency drives)– LMMs as a path forward.– Different energy for process needs with grit suspension. Grit removal LMM may become more common.18

Copyright 2013 by CH2M HILL, Inc. alWBG062211121245ORLDigester Overflows – Causes and Control Measures

Volume Expansion & Density Reductionfrom Gas Holdup20

Can Lead to Scenes Like This!Why?21

Factors with Volume ExpansionPotential Causes Changes in rate orcomposition of feed Changes in mixingregimen Power outage/shutoff ofmixing Inadequate or excessiveheating Rapid pressure dropsPotential Safeguards Pressure relief valves Emergency surfaceoverflows Foam suppression sprays Increased headspace ingas plenum Rapid transfer piping tolower liquid levelNote that some of the safeguards arenonfunctional at reduced liquid density22

Rapid Rise Foam Formation EventFlow Reductionstarts afterShutoff of FeedFlowRateRisingLeg ofEventDecliningLeg of EventDuration,hours23

Rapid Rise vs. Chronic Foaming Rapid-rise foaming:– Frothy sludge with high gas holdup can reduce thedensity to half that of sludge.– Rapid increase in flow rate (as much as 10-fold morethan inflow)– Rapid increase signals operation reaction– Extended decrease in the flow rate that can last aslong as a day– Digesters usually not designed to accommodate Chronic or normal foaming:– A relatively continuous phenomenon– Digesters are typically designed to handle chronic ornormal foaming using such measures as foambreakers with scum nozzles24

Foam Breaker with Scum NozzleVaughan Foambuster (Above) and Scum Nozzle(Below) for control of surface scum and foam buildup25

Foam Management Begins with Start-Up Start-up is a time of transitionTransitions are the most critical periods for foam formationgenerating gas hold-upA proven example start-up protocol is found in the WPCF (nowWEF) MOP 16 “Anaerobic Digestion Manual of Practice No.16”.This document addresses the start-up procedures and is crystalclear on what is required.26

Digestion Process Start-Up MOP 16 and common sense dictate that all systems andequipment must be operational before startup.The monitoring program and laboratory capacity must be ready togo.The staff must be fully trained.A contingency plan must be in place.One person must be in chargeEveryone must buy in to the challenges.27

The alternative outlets for gas & liquid .Pressure Relief Valve& Flame ArresterEmergencyOverflowCan be rendered non-functional due to frothy sludge28

Gas Draw-Off PipeCan be part of the cause of an imbalanced loading29

Frothy Sludge Over the Digester Wall 30

And Onto the Ground31

Control Measures Have an approved start-up plan with buy-insHave a start-up team with a chain of command that meansbusinessAvoid premature start-up at all cost.Avoid complacency at all costVisit other sites where start-ups have been done and capture thelessons learned.32

WBG062211121245ORLAddition of FOG and HSW ConsiderationsCopyright 2013 by CH2M HILL, Inc. al

FOG and HSW Addition Considerations Overall Benefits of Use of FOG as a Resource– Improve sewer and WRRF performance– Provides revenue stream from tip fees– Will generate significant amounts of biogas at low hydraulic loadingin anaerobic digestion– Biogas production when coupled with co-generation can result insignificant cost savings– Biogas use from FOG addition in CHP can reduce GHG footprint andenergy dependence Other Considerations––––Get a handle on quantities and characteristics availableStart slowModeling digester performance/biogas production can easily be doneConsider impact on entire wwtp process train during design34

Key Considerations for Utilizing FOGResources Characteristics and form of FOG or high strengthwastes to be received for co-digestionReliability, consistency and availability of eachmaterial supply as a feedstockCollection network to move the FOG to a receivingstation at the WRRFSpecial requirements for receiving, holding andtransferring FOG on a continuous basis35