Introduction
Spent substrate and root balls going into bin bags every week. These organic materials are potential resources when managed correctly. Plant waste from hydroponic and substrate-based production contains nitrogen, carbon, and structural components that can be converted into resources rather than disposal costs.
Quick Glossary
- Composting — controlled aerobic biological decomposition of organic materials into stabilised organic fertiliser, requiring a balanced carbon-to-nitrogen (C:N) ratio, moisture, and aeration
- Organic production waste — spent substrate (coco, peat, perlite), root balls, rejected greens, plant residues
- Biogas — combustible gas (primarily methane) from anaerobic decomposition; the residual digestate can serve as fertiliser
What and How Much: Typical Hydroponic Waste Streams
Spent coco substrate generates 5–15 kg of waste per plant per season after one or two growing cycles, containing residual fertiliser salts and an embedded root ball.
Root balls and stems from microgreens generate 20–40 kg per week when processing 100 trays per week.
Rejected product makes up a small percentage of yield — direct organic matter without substrate.
Nutrient solution and drainage — liquid waste containing dissolved salts and organic compounds.
Composting: The Most Accessible Route
Composting suits all production waste except pesticide-treated materials with long breakdown periods.
C:N balance is critical. Microorganisms need a carbon-to-nitrogen ratio of 25–30:1. Plant material runs at 15–25:1; wood chips and straw reach 60–100:1. Mixing guideline: combine two parts plant mass with one part dry straw or sawdust for aeration and balance.
Coco substrate at approximately 100:1 requires nitrogen sources: poultry manure, fresh plant material, or drainage water.
Moisture and aeration require 50–60% moisture with weekly turning to prevent anaerobic conditions producing hydrogen sulphide and ammonia odours.
Timelines: 2–4 months with proper management; 6–12 months passively.
Materials to avoid: diseased plants (Fusarium, viruses), pesticide-treated substrate, materials contaminated with chlorine or heavy metals.
Vermicomposting: A Compact Solution
For 5–15 kg of waste per week, vermicomposting with Eisenia fetida produces high-quality vermicompost and worm tea within 4–8 weeks in 100–200 litre containers. No turning required.
Limitations: worms process coco substrate inefficiently; they do not tolerate materials with EC above 3.0 mS/cm. Pre-rinse high-salt materials before introducing them.
Biogas: Requires Scale
Anaerobic processing justifies implementation at several tonnes per month. For small microgreen producers this is impractical; for large agri-complexes with plant and animal waste — economically viable for reducing energy costs.
The resulting digestate serves as fertiliser, closing the loop: waste → biogas → digestate → nutrients for the next production cycle.
Three Mistakes That Cost the Most
- Composting diseased plants together with healthy waste — cold passive composting does not reach pathogen-killing temperatures; separate diseased material or use thermal composting
- Adding unstabilised compost directly to substrate — immature compost consumes oxygen and nitrogen, harming plants; finished compost looks dark and smells earthy
- Ignoring salt content in coco substrate — flush to EC below 1.5–2.0 mS/cm before composting or vermicomposting
Indicators of Success
- All organic material has defined routes (composting, vermicomposting, or transfer)
- Solid waste volume is decreasing
- Compost use is reducing substrate and fertiliser costs
- Chemically treated materials are kept separate from composting cycles