Exercise set
Organics Composting, Anaerobic Digestion, and Digestate Exercises
Solved organics processing exercises for compost blending, pad capacity, contamination, anaerobic digestion, biogas, digestate hauling and release gates.
These exercises focus on source-separated organics, composting and anaerobic digestion. They cover carbon-to-nitrogen blending, moisture basis, pad capacity, aeration, retention time, screening contamination, volatile-solids loading, methane production, digester sizing, digestate hauling and land-application evidence.
Assume simplified screening calculations unless an exercise states otherwise. Release decisions require feedstock audits, contamination controls, odor records, temperature logs, solids basis, biogas quality, digestate testing, storage capacity and outlet documentation.
Release Evidence Notes
Organics evidence must preserve wet mass, dry solids and volatile solids. Many failures come from mixing those bases: a wet tonne is not a dry tonne, and a tonne accepted at the gate is not automatically compost, biogas or land-applied nutrient.
Compost evidence should include feedstock composition, C:N ratio, moisture, pile residence time, pathogen-temperature records, screening losses, contamination and product acceptance.
Anaerobic digestion evidence should include volatile-solids input, organic loading, hydraulic retention time, methane yield, parasitic load, digestate volume, storage and land-application or disposal route.
Engineering Boundary Notes
These calculations do not replace compost process validation, pathogen compliance, air permitting, odor modeling, agricultural nutrient management or digester commissioning. They are screening calculations for organics process release.
Common Release Mistakes
- calculating C:N ratio from wet mass without checking the dry solids basis;
- reporting gross biogas without methane content, parasitic load or flare backup;
- treating screening rejects as invisible rather than residual disposal;
- accepting digestate release without storage, hauling and nutrient loading evidence;
- closing a compost gate from calendar age without temperature and contamination records.
Scenario Map
| Scenario | Exercises | Primary check | Engineering decision |
|---|---|---|---|
| Compost blend and pad | 1, 2, 3, 4, 5, 6, 7, 8 | C:N, moisture, pad area, aeration, retention and product quality | Decide whether composting conditions can be released. |
| Anaerobic digestion | 9, 10, 11, 12, 13, 14 | VS load, methane, power, HRT, OLR and stability | Decide whether digestion capacity and energy claims are credible. |
| Digestate and outlet | 15, 16, 17 | hauling, nutrient area and evidence completion | Decide whether digestate handling is controlled. |
| Release gate | 18 | all-of organics release | Decide whether the organics process can operate or ship product. |
Exercise 1: Compost C:N Blend
Food waste has C:N =18. Yard waste has C:N =45. A blend uses 60\% food waste and 40\% yard waste by dry mass. Estimate blended C:N using weighted carbon-to-nitrogen ratios as a screen.
Solution
Engineering Comment
A blended C:N near 30 is often a reasonable composting target, but the real check should use measured carbon and nitrogen, not only category averages.
Plausibility Check
The answer lies between 18 and 45, closer to food waste because it is the larger fraction.
Exercise 2: Moisture Blend
Food waste is 72\% moisture and yard waste is 38\% moisture. A wet blend contains 8\ \text{t} food waste and 6\ \text{t} yard waste. Compute blended moisture.
Solution
Water mass:
Total wet mass:
Moisture:
Engineering Comment
Moisture near this range can support composting, but porosity and free liquid control still matter.
Plausibility Check
The result is between 38\% and 72\%, closer to food waste because it contributes more wet mass and water.
Exercise 3: Compost Pad Capacity
A compost pad has 3600\ \text{m}^2 available. Windrows require 9\ \text{m}^2 per wet tonne in active processing. Compute active inventory capacity.
Solution
Engineering Comment
This is active inventory capacity, not annual throughput. Throughput also depends on residence time and curing space.
Plausibility Check
Ten square meters per tonne would give 360 tonnes, so 400 tonnes at 9 square meters per tonne is reasonable.
Exercise 4: Pad Throughput From Residence Time
The active composting inventory capacity is 400\ \text{t}. Active residence time is 28 days. Estimate annual active throughput.
Solution
Engineering Comment
The site can only claim this throughput if receiving, curing, screening and stormwater controls have matching capacity.
Plausibility Check
The pad turns over about thirteen times per year, so throughput near 5200 tonnes per year is plausible.
Exercise 5: Aeration Flow Screen
An aerated static pile contains 260\ \text{t} of active compost. The operating target is 0.45\ \text{m}^3/\text{min} per tonne. Compute required aeration flow.
Solution
Engineering Comment
Aeration capacity should be checked against fan curves, duct losses, pile porosity and odor-control routing.
Plausibility Check
About half a cubic meter per minute per tonne over a few hundred tonnes gives a little above one hundred cubic meters per minute.
Exercise 6: Temperature Compliance Fraction
A compost batch requires 72 qualifying hours above the temperature threshold. The logger records 66 qualifying hours. Compute compliance fraction.
Solution
Engineering Comment
The batch is not complete if the permit or product standard requires the full qualifying time.
Plausibility Check
The missing time is 6 hours out of 72, so the fraction should be a little above 90\%.
Exercise 7: Screening Reject Rate
After compost curing, 180\ \text{t} enters screening. Rejects are 14\ \text{t}. Compute reject rate and accepted product mass.
Solution
Engineering Comment
Rejects are a disposal or reprocessing load. They cannot disappear from the recovery mass balance.
Plausibility Check
Fourteen tonnes is less than one tenth of 180 tonnes, so 7.8\% is reasonable.
Exercise 8: Odor Biofilter Empty Bed Contact Time
Odorous air flow to a biofilter is 9000\ \text{m}^3/\text{h}. Biofilter media volume is 150\ \text{m}^3. Compute empty bed contact time.
Solution
Convert flow:
Contact time:
Engineering Comment
Odor release evidence should include airflow, media moisture, pressure drop and breakthrough complaints or measurements.
Plausibility Check
A few cubic meters per second passing through 150\ \text{m}^3 of media gives about one minute.
Exercise 9: Volatile Solids Input
An anaerobic digester receives 42\ \text{t/d} of wet organics at 24\% total solids. Volatile solids are 82\% of total solids. Compute volatile-solids input.
Solution
Total solids:
Volatile solids:
Engineering Comment
Methane calculations should use volatile solids or COD basis, not wet tonnes alone.
Plausibility Check
The VS load must be below the total wet mass and below total solids; 8.27\ \text{t/d} satisfies both.
Exercise 10: Methane Production
Volatile-solids input is 8.27\ \text{t/d}. Specific methane yield is 95\ \text{m}^3/\text{t VS}. Compute methane production.
Solution
Engineering Comment
The yield should be validated with feedstock quality, retention time, inhibition checks and gas-meter calibration.
Plausibility Check
About 8 tonnes per day at about 100\ \text{m}^3/\text{t} gives about 800\ \text{m}^3/\text{d}.
Exercise 11: Biogas Electrical Export
Methane production is 786\ \text{m}^3/\text{d}. Use 9.94\ \text{kWh/m}^3 methane and engine efficiency 35\%. Parasitic load is 650\ \text{kWh/d}. Compute net electrical export.
Solution
Gross electricity:
Net export:
Engineering Comment
Net export is the credible energy claim. Gross biogas energy can overstate performance when mixing, heating and dewatering loads are high.
Plausibility Check
Parasitic load removes about one quarter of gross electricity, so net export near 2.1\ \text{MWh/d} is plausible.
Exercise 12: Digester Hydraulic Retention Time
A digester volume is 1800\ \text{m}^3. Slurry feed is 75\ \text{m}^3/\text{d}. Compute hydraulic retention time.
Solution
Engineering Comment
HRT should be checked with active volume, mixing dead zones and actual feed interruptions.
Plausibility Check
Seventy-five cubic meters per day fills 1800\ \text{m}^3 in about twenty-four days.
Exercise 13: Organic Loading Rate
Volatile-solids input is 8.27\ \text{t/d} and active digester volume is 1800\ \text{m}^3. Compute organic loading rate.
Solution
Engineering Comment
The OLR screen should be compared with the digester type, temperature regime and inhibition history.
Plausibility Check
Several tonnes per day over about two thousand cubic meters gives a few kilograms per cubic meter per day.
Exercise 14: Alkalinity Stability Ratio
Volatile fatty acids are 1250\ \text{mg/L as acetic acid} and alkalinity is 4200\ \text{mg/L as CaCO3}. Compute VFA-to-alkalinity ratio.
Solution
Engineering Comment
This ratio is a stability screen. It should be interpreted with pH, gas production, feed changes and sampling consistency.
Plausibility Check
The VFA value is less than one third of alkalinity, so a ratio near 0.30 is expected.
Exercise 15: Digestate Hauling Capacity
Digestate production is 58\ \text{m}^3/\text{d}. Each tanker hauls 22\ \text{m}^3 and 3 trips per day are available. Determine daily hauling margin.
Solution
Hauling capacity:
Margin:
Engineering Comment
The margin is small. Weekend schedules, land access, weather and receiver limits can turn a positive daily margin into storage overflow.
Plausibility Check
Three tankers at about twenty cubic meters each can move a little more than the 58\ \text{m}^3/\text{d} production.
Exercise 16: Digestate Land-Application Area
Digestate contains 4.2\ \text{kg N/m}^3. Annual digestate volume is 16000\ \text{m}^3/\text{yr}. Agronomic nitrogen loading limit is 170\ \text{kg N/ha/yr}. Compute required land area.
Solution
Nitrogen load:
Required area:
Engineering Comment
Land-application release needs nutrient analysis, soil constraints, seasonal access and records proving where digestate actually went.
Plausibility Check
Tens of thousands of kilograms at a few hundred kilograms per hectare requires hundreds of hectares.
Exercise 17: Organics Evidence Completion
A release checklist has 10 required records: feed audit, contamination log, blend calculation, moisture check, temperature log, odor control, VS load, gas meter, digestate test and outlet record. Eight are complete. Compute completion.
Solution
Engineering Comment
An 80\% completion score is not enough if the missing records are digestate outlet or temperature compliance.
Plausibility Check
Eight out of ten is exactly 80\%.
Exercise 18: Organics Release Gate
A release gate requires C:N between 25 and 35, moisture between 50\% and 60\%, temperature compliance at 100\%, digestate hauling margin above 5\ \text{m}^3/\text{d} and evidence completion above 90\%. Current values are C:N 28.8, moisture 57.4\%, temperature compliance 91.7\%, hauling margin 8\ \text{m}^3/\text{d} and evidence completion 80\%. Decide release status.
Solution
C:N passes, moisture passes and hauling passes. Temperature compliance fails and evidence completion fails:
Release status:
Engineering Comment
The process is close on blend and logistics but cannot release product or close the run without pathogen-temperature and evidence completion.
Plausibility Check
An all-of gate fails when two required records are below threshold.
Validation Package Checklist
- Wet mass, total solids, volatile solids and nutrient basis are kept separate.
- Compost blend, moisture, residence time, temperature and contamination records match the same batch.
- Anaerobic digestion claims include VS input, HRT, OLR, methane yield, parasitic load and gas-meter evidence.
- Digestate release includes storage, hauling, laboratory results, outlet route and land-application constraints.