AD: basic concenpts and principles Flashcards
What is AD
A natural biological process which occurs in swamps, rovers and lake sediments, livestock, rice paddies
process that takes place in landfill to produce landfill gas from depoisited wastes
difference to compost
produced digestate
co2 and ch4 (chemical energy)
what are the biosolids used for anaerobic digestion
garden waste
food waste
animal waste
sewage sludge
what applications are there of biogas
cooking gas boiler for hot water or steam gas engine to drive a generator injection into the gas grid vehicle fuel
what are the different types of anaerobic digestion
- solid concentration
wet digestion: that has a total solid concentration under 15%
dry digestion: has a total solid concentration under 25-40% - Temperature
- mesophilic (35-40C)
- thermocphililic (55-60C) - single stage, two stage of multi-stage reactor
operation mode: batch or continuous
batch
- pros: simplicity of the reactor, low maintance requirements, and minimal capital cost
continuous
- pros: higher treating capacity, stable operation, stable biogas production
Reactor type
Continuous stirred tank reactor
plug flow reactor
- vertical flow reactor
- horizontal plug flow
Anaerobic digestion parameters measured in AD stage
- Hydraulic retention time
(days) : defined as the volume of the digester (m3) divided by the daily feed rate (m3.d-1)
- the retention time is not the same as the length of time the bacteria is in the digester, unless there is a perfect plug flow
- the parameter is useful when considering feedstocks with %DM between about 4% and 12%, e.g. for animal slurry, in which case the optimum retention time will be between 20 and 40 days. - Organic loading rate
- dfined as the mass of organic matter (kg) fed to the digester in one day divided by the volume of the digester (m3).
- a well-desgined anaerobic digester will maximize the organic loading rate, and this will vary according to the feedstock and according to the digester design. A typical figure is between 2.5 & 5.0 kg DM.m-3 d-1
Parameters used for AD
- total solid (kg or tonne, dry)
- solid concentration (%)
- Biodegradable rate (%)
- wet waste (kg or tonne)
- waste (or hydraulic) residence time (days or hours) = V/Q= reactor volume (m3)/ flow rate of waste (m3/day)
- volumetric loading rate = Q/V = flow rate of waste (m3/day)/ reactor volume (m3)
- Organic loading rate= Q/V = flow rate of dry waste (kg.day)/ reactor volume (m3)
- Biomass yield= biogass produced (m3)/ total solid of waste (kg)
- Energy yield= energy (MJ)/total solid of waste (kg)
Biogas production from AD
To predict or measure the methane content, Buswell equation could be used, which is a formula based on chemical composition to predict theoretical yields of component products from digestion
conversion between gas volume, mass and mole
1 mole gas at standards temperature and pressure (STP) = 22.4l
gass mass= gas mole * gas molecular weight
based on ideal gas law, gas volume ration= gas mole ratio
energy unit conversion
1 w = 1 j per second 1Wh = 1 x 3600 j 1kWh= 3600000 j 1 m3 CH4 has heat value of 36MJ 1m3 biogas has heat value of around 22MJ (which depends on percentage of CH4 in biogas)
Hydrolysis
Hydrolytic organisms (bacteria) secrete extra-cellular enzymes to break the bonds on polymetric substances
attach to the substrates using an extracellular polymeric substances (EPA)
organisms have high growth rates but hydrolysis can be rate-limiting depending on the substrate
Hydrolysis: common reactions
Carbohydrates- sugars and alcohols Cellulose - glucose, cellobiose Lignin- degraded very slowly proteins - amino acids, peptides Fats- fatty acids, glycerol
- insoluble organics and complex soluble organics are converted to simple soluble organics
- hydrolysis of carbohydrates takes place within a few hours while proteins and lipids take a few days to break down
fat hydrolysis by lipase simple lipid (triglyceride) is broken down into palmitic acids and glycerol
what are the products produced by acidogenesis?
sugars - fatty acids (energy producers), carbon dioxide, hydrogen
amino acids- fatty acids, ammonia, sulphides, carbon dioxide, hydrogen
Glycerol- Acetate, carbon dioxide
alcohols- fatty acids, carbon dioxide
- fatty acids comprises mainly of (succinate, acetate, propionate, valerate, butyrate, lactate, formate)
what are the characteristics of acidogenesis (fermentation)
fast growth rates (doubling times 2-3 hours)
fastest stages in the process and rarely rate-limiting
often performed by the same organisms that carry out the hydrolytic reactions
enzymatic reactions take place either intracellularly or extracelluarly
- note that hydrolysis can only occur extracellularly as particles are too large to be transported into the cell.
Describe acetogenesis
acetogenic bacteria produce acetic acid, H2 and CO2 from fermentation products
Acetogens fall into two main groups
- hydrogen producing acetogens:
e. g. butyrate and propionate - homoacetogens
not fast growth rates
sensitive to their environment (pH, temperature, hydrogen potential, pressure)
Syntrophic relationship with methanogenic archaea
note that all processes are interwined and you have to provide optimum conditions for each
polymers> monomers> FA> acetetic acid> Ch4 + CO2
Methanogenesis
this is the terminal step in AD producing biogas a mixture of CO2 and CH4
- Methanogens have slow growth rates (doubling times ~ 6-10 days
Sensitive to physical conditions (such as temp, pH, inhibitors)
work in synergy with acetogenic organisms (inter-species hydrogen transfer)
Considered to be some of the oldest organisms on the planet, from a time the atmosphere was anoxic
Methanogenic routes
one groups produces methane from acetate
one groups produces methane from H2 and CO2
methane can also be formed from formic acid
HCOOH = CO2 + H2
4H2 + CO2 = CH4 +2H2O
one stage vs two stages
in a two phase system, hydrolysis and acidogenesis occur in one stage and acetogensis/ methanogenesis occur in stage 2
- first stage is less sensitive to pH
- optimize mixing, retention times, loading rates (gmVSS/m3)
- better control of methanogensis which is more sensitive
what are the three main groups of microbes
fermentative acid-producing bacteria are relatively fast growing and under ideal labarotory conditions can double their mass every few hours providing they have the right food supply and environmental conditions
hydrogen-producing acetogenic bacteria are also relatively rapid growers under ideal conditions but are highly sensitive to environmental conditions
methanogenic bacteria are extremely slow growing and are only capable of doubling their mass every few days
What affects growth rate
this is the most important criterion to consider in relation to biomass retention in reactor-based systems
a conventional anaerobic digestor is a once through system i.e. there is NO biomass recycle
if the retention time is shorter than the maximum specific growth rate (u max) of a group of micro-organisms then members of that groups will be washed out faster than they can grow.
this is a fundamental rule in the continuous culture of microbes
what is the growth rate of Methanogens
u max is between 6-10 days
- we must therefore operate a single pass digester to have a retention time greater than 6 days under optimal conditions
- we also want a safety factor and therefore it would be unusual to find a digester working at a retention time less than 12 days
what are the optimal conditions for methanogens
higher temperatures, higher growth rate, lower retention time
pH: 7 (note this is a very narrow range)
explain the importance of ammonia levels
Toxicity depends on pH
- alkaline conditions max 150mg/l
- acid conditions max 3000mg/l
- important as methanogens can only work in a narrow pH range
Volatile fatty acids
Only propionic is known to have toxic effects on methanogens at neutral pH
VFA concentrations should be <1000mg/l and higher levels may indicate the process is not optimized. It is thought however that pH is usually more critical than the actual VFA concentrations
inhibition: sulphates
Sulphur is present in all biological materials especially those containing high concentrations of protein
sulphates present in the feed material are reduced to hydrogen sulphide by sulphate reducing bacteria (SRB)
SRB compete with methanogenic organisms for the same substrates
result
primary competition is for common substrates (reducing CH4 output)
secondary inhibition as a result of toxicity of H2S produced.
Hydrogen sulphide is very toxic to M.organisms and results in inhibition
effect of digester imbalance or reduction in methanogenic activity
methanogenic populations are by far more sensitive to a number of parameters than the acidogenic bacteria
- is conversion of methane pre-cursors slows down due to the inhibition or slowing down of the methanogenic population
- acid intermediate products accumulate
- digester pH falls
- H2 and CO2 start to accumulate
why is the accumulation of hydrogen particularly important?
this inhibits the process of acetogenesis and there is no further conversion of the longer chain acids to acetate (feedback inhibition)
there is a build up of long chain fatty acids and a drop in pH
at this point the process is regarded as being stuck as well as being sour
example of a simple condition that could lead to a digester failure
malfunctioning of heat exchanger causes digester temperature to drop and the conditions for several days
overall a drop in biogas productivity as the rate of activity of the methanogens and acidogens are both effected
pH remains unchanged
possibly a build up in unhydrolysed material is digester continues feeding
however: there is a rapid return to normal operating temperature when the heat exchanger is fixed
- because of their fast growth rate the acidogenic bacteria respond much more rapidly than the methanogensis
- rapid production of acids and a drop in pH
- methanogens affected by low pH and because of their slow growth rate fail to re-establish themselves
- reactor goes sour
