ABE 482—Environmental Engineering in Biosystems

September 22

Lecture 8

Compost Recipe Example 3

� How much water should we add to our piles to reach 60% wet basis moisture content in each one?

Summary of Composting

� Composting is “controlled” decomposition

� Weight and volume reduction…but requires effort!

� Nitrogen: what you want to get rid of

� Carbon: amendment materials to mix with nitrogen

� Must balance C:N, moisture, oxygen

� Monitor temperature, airspace, density

� Compost is “done” when temperature doesn’t rise after turning

Today (Anaerobic digestors)

� Quick definition

� Applications and types of anaerobic digesters

� Types of anaerobic digesters

� Biogas production

� Sizing of digesters

� Mass and energy balance for anaerobic digesters

What is anaerobic digestion?

� Methanogenic and acidogenic breakdown of

complex organics

� Oxidation of organic or inorganic compounds in the absence of oxygen

� Produces biogas and less biomass than aerobic digestion

Applications of anaerobic digestion

� Any wet organic material

� Waste paper, grass clippings, foodwaste,

sewage, animal wastes

� Energy or silage crops

� Wood waste?

� Livestock production facilities

� Wastewater treatment facilities

Anaerobic Digester for Livestock Production Facilities

Production Building Digester

Lagoon

Biogas

Flare

Generator

HotWater

Manure Bypass

Composting orland application

DigesterProduction buildingGreenhouse

DigesterProduction buildingPower grid

Liquid fertilizer

Solid/LiquidSeparator

Anaerobic Digester for Wastewater Treatment Facilities

Types of Digesters

Types of digesters

� Low rate conventional digester� Digestion time = 30-60 days� Intermittent mixing, feeding and sludge

withdrawal� Floating cover or fixed cover

� High rate digester� Digestion time = 10-20 days� Continuous mixing, continuous or periodic

feeding and sludge withdrawal� Fixed cover

� 2 stage digester� High rate followed by a low rate

Biogas production

� Composition: 65-70% CH4, 30-35% CO2, water vapour, trace amounts of H2S, H2, N2

� Quantity: 810-1120 L of digester gas/kg of volatile or organic solids digested

� Chemical balance (carbs�sugars�methane and carbon dioxide)

C6H12O6 � 3(CO2) + 3(CH4)

180 g/mol 46 g/mol 16 g/mol

Factors controlling the conversion of waste to gas

� The type of waste being digested,� Its concentration,� Its temperature,� The presence of toxic materials,� The pH and alkalinity,� The hydraulic retention time,� The solids retention time,� The ratio of food to microorganisms,� The rate of digester loading,� And the rate at which toxic end products of digestion are removed.

Sizing of digesters

� For CSTR digesters, required volume = flow rate * hydraulic retention time

� Volume of biogas = 5-15% tank volume

Example 1Sizing of digesters

� Determine the volume of CSTR digester required to digest pig slurry from a farm with 2,000 pigs. The hydraulic retention time is 15 days.

Mass and Energy Balance

� Mass balance

� Total mass of inputs = total mass of

outputs

� Inputs = feedstock

� Outputs = digestate and biogas

� Feedstock

� % dry matter (%DM)

� % organic dry matter (%ODM)

� % organic matter reduction (%OMR)

Feedstock

� % dry matter (%DM)� %DM = dry mass/wet mass� %DM = 1 – MCwb� Dry material at 105°C for 24 hours

� % organic matter (%ODM)� %ODM = 100 – Ash%� Ash% = non-volatile organics/dry mass� Cook at 550°C to drive off volatiles

� % organic matter reduction (%OMR)� %OMR = (%ODMinput-%ODMoutput)/%ODMinput

� 30% for anaerobic digestion of cattle manure� 70% for some energy crops

Simple Feedstock Calculation

� Feedstock� Mass (rate) = 50 tonnes/day

� %DM = 30%

� %ODM = 90%

� %OMR = 66.7%

� Calculate:� Mass (rate) of dry matter and organic dry matter in input

� Mass (rate) of organic matter reduction

Simple Mass Balance

Input -OMR

ODM

Same as input

OMRSame as input

Same as input

TotalBiogasAshWater

Input

Output

Water + ash + organic

OrganicDry -organic

Total – dry

TotalODMAshWater

Digestate

Simple Mass Balance

Mass of feedstock = 50 tonnes/day

Dry matter of feedstock = 15 tonnes/day

Organic dry matter of feedstock = 13.5 tonnes/day

Organic matter reduction = 9 tonnes/day

Mass of digestate = 41 tonnes/day

Calculate:

%DM and %ODM of digestate

DM and ODM rates

4.5

ODM

5091.535

TotalBiogasAshWater

Input (tonnes/day)

Output (tonnes/day)

5013.51.535

TotalODMAshWater

Anaerobic Digestion Parameters

� Consider the Continuous Stirred Tank Reactor (CSTR)

� Hydraulic retention time

� Specific loading rate

� Specific biogas production

� Specific methane production

Next Day

� Anaerobic digester parameters

� Biogas production

� Digester heat balance