Anaerobic Digestion

Anaerobic Digestion

Outline

What is it? Purpose? Wastewater Treatment.

Mechanism of Anaerobic Digestion – Microbiology + Some Definitions

Application. When is it useful?

Process Requirements/ Process Equipment

Process Design & Consideration - Some Equations

Anaerobic Process Operation – Indicators and Causes of Imbalance

References

Malina J.F. & Pohland F.G., Design of Anaerobic Process for the Treatment of Industrial and Municipal Waste (TD755.D457)

Grady C.P. & Lim H.C., Biological Wastewater Treatment (TD755G733)

Wheatley A, Anaerobic Digestion: a Waste Treatment Technology (TD796.5.A532)

Kiely G., Environmental Engineering , 1997, Irwin/McGraw-Hill

Prof. Bungay’s Website

Well, what is it?

Define Anaerobic Digestion

The use of microbes in the absence of oxygen for the stabilization of organic material by conversion to methane, carbon dioxide, new biomass and inorganic products.

Used in treatment of wastewater: Industrial, agricultural, municipal

Okay, what for then?

Reducing the solids content and volume of the wastewater stream. Approximately 25-45% of feed destroyed

Stabilization – decomposing highly putrescible organic matter to stable or inert organic and inorganic compounds

Think government regulations, cost minimization

Although widely used in wastewater treatment, the proper procedures for selection, design and operation of anaerobic processes are not well understood by many wastewater treatment professionals.

Right, why would I prefer A/D?
Advantages:

Production of methane gas (potential source of fuel)
No oxygen requirement
Low nutrient requirement
Low production of biological sludge (low biomass/cell growth)
High treatment efficiency for biodegradable sludge
Inoffensive residual sludge. May be used as soil conditioner
Pathogens are inactivated. Concentration reduced

How does the mechanism work?

First note:

Not all volatile solids in the waste stream are biologically degradable. 30–40% wt of initial remains after anaerobic digestion

See Diagrams

Figure 2.1. - Detailed

Figure 5.2. – Simpler

Although sequential in nature, acid and methane fermentation takes place simultaneously and synchronously in well-buffered, active digesting system

Maintain a balance between rates of acid and methane production important

3 possible steps that is rate limiting.

Depends on the characteristic of the wastewater.

High particulate concentration or high soluble organic concentration

Digestion retention time should be long enough to accommodate.

Think microbes. Keep them happy by keeping healthy environment.

This depends on

Temperature
pH
Alkalinity
Presence of Inhibitors (Toxicity)

Maintain conditions optimum for rate limiting bacteria

Now that’s all good. But when does A/D apply? How do I know when I can use A/D technology?

(Oh you curious engineer!)

Whatever the case, the following two are governing:

Must be economically advantageous

Must achieve wastewater specifications required

Understand, not always stand-alone. Combinations with other treatment technique. Constitute only PART of treatment system.

Factors to Consider for Screening Applicability:

Origin and Nature of Wastewater

Wastewater Concentration

Most suitable for wastewater with biodegradable COD in intermediate to high strength range

Organic removal efficiencies increase with increasing wastewater concentration. Dilute wastewater, BOD load reduction is insignificant (50%).

Table 2.4

Temperature of Wastewater

Higher temperature, shorter digester retention time

As low as 8 C

Mesophilic (30-40 C)

Thermophilic (50-60 C) –Unstable though

Depending temperature of wastewater discharge, heating may be necessary. Otherwise provide longer retention time (at larger reactor/capital cost!)

Concentration of Suspended Solids (Insoluble Organics)

High concentration of Insoluble, hydrolysis rate limiting. Need high retention time – 10 to 20 days. Otherwise, reduce organic loading into digester (More next time)

High soluble organics can use high rate anaerobic digestion. Retention time several hours only. Careful with entrapment of solids, can dilute active microbial biomass

Presence of Toxic Compounds

Can inhibit microbes.

Depends on nature of substance, concentration and degree the process has become acclimatised to. Microbes can develop tolerance

See Table 2.5 & 2.6. Compare known concentration with table and assess if process inhibition a serious problem

Expected Treatment Efficiencies

Must meet the specification

Laboratory testing essential to establish this

Mass Discharge Rate of Organic Pollutant

Cost competitiveness of anaerobic process improves as total mass discharge rate of biodegradable COD becomes larger

Use COD for characterising of organic strength rather than BOD because BOD tends to underestimate for anaerobic conditions. COD covers both biodegradable and non-biodegradable however.

So my waste stream can apply A/D. Right, what are the process requirements for such a system?

Conditions of the system must be optimum to maintain balance of the microbial groups; hydrolysis, acid and methane producing bacteria.

See Table 5.3

Temperature:

Specific growth rate of microbes increase with increasing temperature in a relatively narrow band

Minimum retention time decreases with elevated temperature, hence reactor size

Low temperature okay, if long enough residence time allowed

Typical operate in mesophilic region: 30 to 35 C. Thermophilic unstable and costly.

pH:

Methane producing bacteria sensitive to pH. Inhibited at low pH, less than 6.2. Volatile acids accumulate – further drop in pH

Consensus to maintain operating pH near neutral, 7.0

Toxicity:

Effect on growth rate of bacteria by inhibiting (declined).

Consider :

volatile acids - propionic acid limiting

ammonia – from protein degradation. Free NH₃ more toxic ( toxic if > 150mg/l). pH to control. At pH 7.2, mostly ammonium ions (toxic if 3000mg/l)

metal ions – from addition of base to control pH. Careful not to exceed toxic levels. No sudden increase and allow acclimatisation period, can tolerate moderately inhibitory for some time.

Sulfides – from protein degradation. Only soluble sulfides effect. Precipitate out by addition of iron.

Heavy metals – toxic at low concentration. Not too much worry, precipitate out with sulfides ie no longer soluble. Naturally occurring usually sufficient.

Rapidity of Changes:

Methanogenic bacteria has lower growth rate compared to non-methanogenic bacteria, hence slower responses to change.

Changes to reactor environment should be made at rate tolerated by methanogenic bacteria ie slowly and in slow steps. Allow for acclimatisation.

Loading:

Is the mass of organic matter added per unit volume per unit time

Given, loading should not exceed 6.4kgVS/m³/day

High loading higher ammonia level which may be toxic.

More later in design (will revisit)

Okay, what systems are available? Basically, what process equipment am I looking at?

A/D Technology Available:

See Figure 2.6.

See Table 2.8.

Process Equipment:

Reactor Tanks …

Temperature Control/Heating …

Mixing …

More next time ….

Some review and maybe clarification

Two sludges from waste water stream to be aware of:

Industrial sludges (their waste)

Municipal sludges (our waste)

Objectives of wastewater processes are to:

Reduce BOD, reduce TSS, reduce N & P, and reduce pathogens.

Organics in wastewater stream will deplete DO in waterways, N and P can lead to eutrophication.

Anaerobic treatment is part of the treatment process for the wastewater stream. See Figure 12.6 (municipal) and 12.7 (industrial) for schematic for wastewater treatment plant.

Anaerobic digestion (A/D) is part of the sludge treatment facility, treating sludge from the primary and secondary treatment.

Primary treatment

clarification, sedimentation, settling

solids separation by settlement and improves wastewater quality for secondary treatment

Secondary treatment

biological treatment, reduction of BOD in the clarified waters/suspended solids.

A/D can also act as a first stage treatment of high strength organic wastes. That is, it reduces the organic loads to magnitude of COD that can be accommodated in conventional aerobic processes.

Basically:

A/D converts the biodegradable material in the wastewater stream to final products of methane, carbon dioxide, biomass (little) and inorganics. Through this conversion, it reduces the solid content/volume and COD of sludge and stabilises the sludge by degrading the putrescible organic matter.

A/D is not a complete processor of wastewater on its own. It is an addendum to the process.

Books seem to use Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) interchangeably (and hence I do too).

But differentiate that:

BOD – measures the amount of oxygen used by bacteria in degradation of organic matter

COD – measures the oxygen equivalent of organic matter contents in wastewater susceptible to oxidation by strong chemical. Contains both biodegradable and non-biodegradable.

And typically, COD removal = 1.5 x BOD removal (Ref 1)

RECALL:

Three Mechanisms Occuring:

Hydrolysis Process – conversion of insoluble high molecular compounds (lignin, carbohydrates, fats) to lower molecular compounds

Acidogenesis Process – conversion of soluble lower molecular components of fatty acids, amino acids and sugars (monosaccharides) to lower molecular intermediate products (volatile acids, alcohol, ammonia, H₂and CO₂)

Methanogenesis Process – conversion of volatile acids & intermediate products to final product of methane and CO₂

(I have included another schematic of methane production, Fig 13.1 which I thought is a good descriptive figure)

Considerations for the A/D Process:

Wastewater Characteristic – COD, Suspended Solids

Temperature

Toxicity

Treatment Efficiency

Application Ranges:

< 50mg/l COD à carbon absorption

50 – 4000mg/l COD à Aerobic Digestion

>4000mg/l COD à Anaerobic Digestion –High Rate/Low Rate

> 50,000mg/l COD à Evaporation & Incineration, Wet Combustion

And Today’s "Extremely Exciting" Topics are:

Process Design

Operations – Monitoring & Problems

What happens to the treated sludge later?

I’m an engineer. Where’s the maths? How is the process design like?

First some definitions & equations:

Volumetric Organic Loading Rate, B_v = C_i(Q/V)=C_i(1/t) Eq (1)

Where S=Wastewater Feed Concentration

Q=Wastewater Feed Flowrate

V=Bioreactor Volume

t= Space Time

C_i expressed as the mass of Volatile Solid (VS), COD, BOD ..

Depends on method of assay (and which books!)

Solid Retention Time, SRT = Mean Cell Residence Time, MCRT

= 1/m (Ref 1)

is the average time microbes remain in the system

Inverse relationship with specific growth rate

(During inhibition, specific growth rate decreases, hence SRT to accommodate this growth rate increases)

Hydraulic Retention Time, HRT = V/Q = t

is the average time the sludge remains in the system

Something about reactor configurations:

See Figure 13.2, 13.3 and 13.4

a) Two types: high rate and low rate digesters.

Low rate characterised by intermittent mixing, sludge feeding and sludge withdrawal or no mixing completely (stratified).

High rate characterised by continuous mixing. Shorter retention time (hence name!)

Two generations of reactors: First generation and Second generation

First generation, HRT = SRT (no recycle, ie biomass concentration)

Second generation, HRT not equal to SRT

See Figure 2.6 and Table 2.8

Included just to give an idea of the anaerobic treatment technologies available.

Basically:

Suspended growth- good for wastewater containing high concentration of particulate biodegradable material

Supported growth- wastewater containing mainly soluble organics

Hybrid - for intermediate levels of particulates

Process Design Considerations include the following:

Solids Retention Time (SRT)

Hydraulic Retention Time (HRT)

Organic Loading Rate

Gas Production

Tank Configuration

Mixing Systems

Heating Systems

Design criteria:

Want retention time of sludge sufficient to achieve the specific reduction in volatile solids content (digestion) in the digested sludge.

This degree of digestion depends on the ultimate disposal of the digested sludge (more later). Also the organic loading rate, ie the concentration of solids in the feed controls the volume of the reactor.

Basically,

Retention time affects extent of destruction of volatile solids and size of digestion reactor. In turn the size of digestion reactor and concentration of solids in the wastewater feed dictate the solid loading that the digester can have and still maintain the required minimum hydraulic detention time.

Note the relationship. A specific reactor size and hence retention time, will tolerate up to a certain loading before it quits (goes sour). KEEP LOADING BELOW THIS LIMIT.

Recall: SRT refers to the microbes whereas HRT refers to the sludge

High SRT result in process stability and minimal sludge production, whereas low HRT reduces reactor volume and capital cost.

Another objective is to maximize the reactor microbe/biomass concentration. (More microbe, more degradation process) Lead to development fixed film reactors with inert medium to hold biomass. The traditional CSTR model, recall that biomass will be loss on withdrawal of effluent. Also can recycle stream to disconnect SRT from HRT, by returning biomass to process stream.

Note: HRT=SRT if no recycle. Design as CSTR without recycle conservative, because SRT_min is highest. To maximize removal capacity, the SRT is maintained as high as possible. High SRT also provide buffering capacity.

I found two possible design approaches:

Based on SRT

Based on volumetric organic loading rate, B_v

In both cases, preliminary laboratory studies were required.

Based on SRT (Ref 2)

Obtain data comparing degree of volatile solid destruction against SRT

Select SRT to match specifications.

Ensure SRT selected is above SRT_(min) =1/m_max

(ie, enough time for microbes to grow)

For reactor with no recycle SRT=HRT=t

Know flowrate, determine reactor volume, V

Calculate loading (Eq.1) and check not exceeding limit specified on Fig 21.10

If exceeding limit, need to have more dilute inlet feed (alter Q) or have a higher SRT

No recycle conservative for design because lower concentration of microbes (biomass) in the system. SRT highest.

Recycle ratios typically 2 to 4 employed. Use mass balance and SRT, to establish HRT.

Based on volumetric organic loading, rate, B_v (Ref 1)

Obtain data comparing the removal efficiency of process as a function of organic loading rate.

With C_i and Q measured and B_v selected, V can be estimated using Eq.1

(This is odd, there was no mention of the SRT. I assume, in their laboratory testing, a SRT was chosen and efficiency as a function of organic loading was studied)

A conventional digester: Is like the "CSTR" (But in practise it doesn’t work like the CSTR, ie not perfect mixing)

Design considerations mentioned here apply to a conventional digester.

Let’s look at SRT (since it’s an important design parameter):

In all application, SRT must be maintained at or above the minimum value to prevent washout and provide a margin of safety in the event of transient inhibition of reactor biomass or increased loading.

In comes our fudge factor!

Ref 3 established critical retention time = 5 days, and suggested safety factor of 2.

Ref 1 mentions SRT required range from 4 to 10 days, and suggest design safety factor up to 3, ie conventional digester have 12 to 30 days.

Finally, some typical figures used as design parameters:

a) Loading:

between 3.2 to7.2 kg VS/m3day (Ref 1, pg 179)

should not exceed 6.4 kg VS/m3day (Ref 2, pg 854)

between 2 to 3 kg VS/m3day (above 3.2 kg Vs/m3day toxicity of ammonia limiting) (Ref 4, pg 569)

for conventional digester (CSTR, no recycle), loading rates of

to 6.0kg VS/m3day typical or 1 to 10 kg COD/m3day

b) SRT:

Wastewater high in particulate matter (insoluble), hydrolysis step rate limiting, min SRT is 4 to 10 days (Ref 1, pg 62)

Wastewater high in soluble organics, methanogenesis step rate limiting, min SRT is 2.5 to 5 days (Ref 1, pg 62)

Municipal wastewater sludge SRT no less than 10 days (Ref 3, pg 569)

The reactor:

Typically cylindrical with diameters of 5 to 50m and heights 3 to 25m made of steel. Bottom slopes (min 6:1) to the centre to concentrate digested sludge.

Europe likes the eggs-shaped digesters.

See Figure 5.10 & 5.12

Once reactor is sized, heating and mixing system are considered.

Mixing Configuration:

Some natural mixing from rising gas and thermal convection currents.

Figure 4.2 (Ref 3)

Required to:

prevention of surface scum layer (initial)

eliminate thermal stratification and maintain uniform temperature

disperse feed wastewater within active biomass

eliminate dead zones and isolated pockets

Basic methods:

recirculation of sludge

recirculation of gas through sludge

internal mechanical mixing

gas recirculation most economical

Design - calculate power cost requirement for mixing configuration

Heating Configuration:

Important to maintain temperature as constant as possible. Sharp fluctuation detrimental for methane forming bacteria especially. Fluctuation kept to +/- 1 C/day.

Optimum growth temperature range.

More costly for the thermophilic range.

Fouling a problem, heat exchangers must be cleaned regularly.

High concentration of solids, heating requirement reduced from less water.

Basic methods:

external heat exchangers - concentric tube exchangers

steam injection

direct combustion

Design - calculate heat requirement ie heat to heat feed wastewater sludge to desired temperature and compensate heat losses

Gas Storage:

Recall methane is formed.

Need to be stored prior to usage.

Traditionally reactors fitted with floating covers that rise and fall with production

Typical gas production rates - 0.75 to 1m3/kg VS destroyed

60% methane, rest is CO2

Gas recovery system for reuse or gas recirculation for mixing

Nutrient:

Sufficient in the feed wastewater stream. Nutrient deficiency is remote.

A Typical Setup for Municipal Waste Treatment

The 2 stage digester:

Before disposal, anaerobic sludge will undergo some type of solids separation and concentration. Hence the 2 stage digester configuration.

See Figure 5.6

First stage is heated and mixed. Active biodegradation.

Second stage digester function to gravity concentrate the digested sludge (effluent) and thereby reduce the volume of sludge requiring disposal by separating out the supernatant. No mixing or heating. Little solid degradation but gas trapped in sludge slowly realised.

Second stage digester can also function as standby digester capacity and storage of digested sludge. Enables flexibility to cope with changing loading rate. Therefore, it should be designed with heating and mixing capabilities (a question of capital cost!).

Be aware:

Digested sludge does not thicken easily because continued gas production and flotation by sludge saturated by CO₂ and CH₄ as well as high proportion of fine sized particle

Supernatant fluid contains a considerable amount of volatile solids and organic constituent. Return of it to head works of treatment facility impacts performance of other treatment process adversely. Add to suspended solids, N and P and oxygen demand in the wastewater.

Recycled supernatant can result in solid build-up in the plant and hence increase in final effluent suspended solids.

Also C, P and N returned to the liquid process, hence build-up into effluent stream.

Therefore separate biological, chemical and physical treatment of supernatant may be need. Depends on regulations. See Table 5.8 for some process alternative.

My A/D has been built. Do I need to monitor the operation?

Unbalanced anaerobic treatment indicated by increase in CO2, decrease in methane and increase in volatile acid concentration.

Unbalanced from changes in loading, temperature and composition of sludge.

Sufficient time for microbes to acclimate to new environment can help remedy.

Imbalance more frequent in starting up new digesters.

Basically : Sour digesters are bad.

Control digester upset by reducing loading and maintaining pH near neutral and time to reacclimate.

Recall process difficulties arise from differences in growth rates of the two main microbial group, their responses to changes in organic matter and sensitivity to pH and temperature.

Indicators of Reactor Performance.

No single parameter that can be isolated as best indicator.

Historically, rate of gas production. However, does not describe environment within digester and usually too late.

The failing digester would trigger a sharp increase in free volatile acid, carbon dioxide content of gas and decrease in pH.

Parameters that provide insight into condition early enough are:

volatile acids concentration

alkalinity

CO₂ content

Alkalinity:

From the carbonic acid-CO₂system.

Indicate buffering capacity of system. Higher alkalinity, more stable pH and tolerance to fluctuation in volatile acid concentration.

Daily evaluation of:

Volatile acid concentration -> direct titration

Alkalinity -> direct titration

pH -> electronic pH meter

CO₂-> gas chromatography or chemical reaction with sodium hydroxide

Temperature -> minimize temperature fluctuation (for biomass activity)

The well behaved digester:

Volatile acid should be low < 100mg/l (Ref 3), normal 200-400mg/l (Ref 4) Mainly acetic acid if stable. Stressed digester butyric and propionic will increase. Some volatile acid can be inhibitory.

pH at 7.0 maintained

ratio of volatile acids to bicarbonate alkalinity <= 0.4. Can accommodate moderate volatile acid fluctuation

Causes of Imbalance

Increase in loading from increase in concentration or flowrate of wastewater feed.

Change in reactor temperature

Decrease in effective reactor volume (hence SRT) through solid deposition

Toxicity from feed wastewater stream

The Fate of the Digested Sludge.

Dried in a drying sand bed or evaporation basin. Leachate collected and returned to plant.

Dried sludge can be:

Used as soil conditioner.

Mixed with bulking agent and composted

Bury into landfill

Incinerated