Field Guide

Before you start — safety checklist

Read through this before you dig. If you cannot honestly tick every item, do not build the bed yet.

• You understand that the bed reduces load but does not make water safe to drink.
• You have arranged a verified disinfection method — provided and instructed by your local health authority or a qualified water professional — for any water you intend to drink. See verified treatment categories.
• The site is at least 30 m (100 ft) from any drinking-water well, spring, cistern, or surface source you draw from, or further if local rules require it.
• The site is downhill of any drinking-water source — the bed cannot drain back toward water people drink.
• The inflow is not from a known industrial, mining, fuel, pesticide, or hazardous-waste source.
• You have a plan to keep children, infants, pregnant women, older people, immunocompromised people, pets, and livestock away from the bed and its output. See Safety & Limits.
• You will post a clear sign in the local language: "Pre-treatment bed — do not drink, do not eat mushrooms here."
• You have written down local emergency, clinic, and water-authority contacts and kept them where anyone can find them. Where to get help.
• You have talked to a local health worker, water professional, or community elder about your plan if any of the above is uncertain.

What this is

• A low-cost, DIY layer that helps reduce organic load and trap sediment in flowing water.
• A way to slow runoff so it has a chance to settle and be partly broken down by fungi.
• A teaching tool for communities learning about ecological water systems.

What this is not

• Not a drinking-water treatment system on its own.
• Not a guarantee against bacteria, viruses, or parasites.
• Not appropriate for industrial discharge, fuel runoff, or chemical contamination.
• Not a replacement for boiling, chlorination, UV, ceramic filtration, or a certified water system.

Materials (use what you have locally)

• Shovel and a bucket or watering can.
• Washed gravel, 5–20 mm grain (drainage layer; do not use limestone if your inflow is acidic).
• Coarse, washed sand, ~0.5–2 mm grain — effective size (d₁₀) roughly 0.3–0.5 mm gives the best balance of filtration and flow.
• Straw, dry leaves, or other dry plant matter (surface mulch; supplies carbon and limits desiccation).
• Hardwood chips, freshly cut, ~1–5 cm — oak, maple, beech, alder, willow, poplar, eucalyptus all work. Avoid aromatic softwoods (cedar, juniper, pine, fir): their resins and terpenes inhibit colonization.
• Mushroom spawn from an identified strain. A piece of colonized local wood can work only when a local mycologist or experienced grower has identified the species as a safe wood-decomposer (saprotrophic) fungus.
• Simple flow barriers: stones, logs, or scrap planks to keep water inside the bed.
• Optional: a length of pipe or guttering to direct water in.

Recommended fungi

Choose a vigorous white-rot or wood-loving species. White-rot fungi secrete extracellular ligninolytic enzymes — laccases and class II peroxidases (manganese peroxidase, lignin peroxidase) — which depolymerize complex organic matter and many aromatic pollutants.

• Oyster (Pleurotus ostreatus): the most common mycofiltration species. Fast colonization, broad substrate tolerance, well-documented laccase and manganese peroxidase activity. Good first choice in most climates.
• Wine cap, also called king stropharia (Stropharia rugoso-annulata): the strongest performer for outdoor wood-chip beds in temperate regions; thrives in straw–wood-chip blends and tolerates seasonal cycles.
• Turkey tail (Trametes versicolor): slower to establish but has unusually strong ligninolytic and aromatic-pollutant degradation activity in the literature.
• Local saprotrophic fungi (decomposer fungi that feed on dead plant matter): only when guided by a local mycologist, university extension, or experienced grower.

Whatever spawn you use, never eat mushrooms that fruit on a bed receiving runoff or greywater, and never assume a wild fungus is safe without expert identification.

Build it

1. Pick the flow point. Find where dirty water already collects or moves — a low spot, downspout, or slow drainage line. Stay at least 30 m (about 100 ft) from any drinking well, spring, or open water source you draw from. Use a larger distance if local rules require it.
2. Dig a trench or build a raised bed. Roughly 30–60 cm wide, 30 cm deep, and as long as you can manage. Slope it gently (≤1 %) so water moves through, not over the top. Size for a target hydraulic loading rate of about 0.05–0.5 m³ per m² of bed surface per day — slower flow gives more contact time and better treatment.
3. Add gravel. A 5–10 cm drainage layer at the bottom so the bed never sits in stagnant water.
4. Add sand. A 5–10 cm layer of washed coarse sand on the gravel for sediment capture.
5. Add wood chips and mycelium. Mix spawn (or chopped colonized wood) into a 15–25 cm layer of fresh hardwood chips at roughly a 1:10 spawn-to-chip ratio by volume. This is where most of the biological work happens. Aim for a hydraulic residence time of at least 10 minutes through this layer at design flow.
6. Cover with straw or leaves. A 3–5 cm mulch holds moisture and protects the mycelium from drying sun and UV.
7. Guide the water through, not around. Use stones or logs to channel inflow into one end and outflow from the other. A perforated pipe under the chip layer can help distribute flow evenly.
8. Keep it damp, not flooded. Mycelium needs moist, well-aerated substrate. Standing water drives the bed anaerobic, kills the fungi, and produces sulfide odours.

Allow 3–6 weeks of moist, unloaded "establishment" time before sending dirty water through the bed, so the mycelium can colonize the wood chips and form a hyphal mat.

Layer view

What to expect — and what not to

Field studies of mycelium-wood-chip biofilters report meaningful but variable load reductions. Realistic targets for a well-built, properly loaded bed:

• Turbidity and total suspended solids: typically reduced by ~50–90 %, mostly via the sand and gravel layers.
• Biochemical oxygen demand (BOD₅) and chemical oxygen demand (COD): typically reduced by ~30–70 % over weeks of operation.
• Indicator bacteria (E. coli, total coliforms): commonly reduced by 1–2 log₁₀ units (90–99 %), with high variability between sites and seasons. This is not pathogen-free water.
• Nitrogen, phosphorus: partial uptake; full nutrient polishing requires a downstream plant buffer (see Design C).

Performance falls when the bed is overloaded, when residence time is too short, when the substrate is exhausted, or when the inflow shifts (a fresh manure pulse, an upstream spill). Treat the numbers above as plausible ranges, not guarantees.

Maintenance

• Inspect after every heavy rain. Look for bypass channels around the bed and patch them with stones or soil.
• Top up dry straw or leaves if the surface is bare or eroded.
• Replace organic matter (wood chips and straw) every 2–4 months, or sooner if it compacts and stops draining.
• If you smell rotten eggs or sour decay, the bed is anaerobic (no oxygen reaching the material) — add dry matter and improve airflow.
• Note flow before and after rains using the water testing log.
• Keep livestock and children from playing in the bed itself.
• Stop using the bed and seek expert advice if you notice a fuel or chemical smell, a known upstream spill, an animal die-off near the source, or sudden discolouration of the inflow.

Take it offline

The Field Guide is also available as printable pages for use without internet access.

Open printable Field Guide