Safety & Limits

What it may help with

• Reducing organic load (decaying plant matter, animal waste).
• Capturing sediment and slowing fast runoff.
• Lowering biological oxygen demand (BOD), which makes downstream disinfection more effective.
• Reducing indicator-bacterium counts (typically 1–2 log₁₀ reduction in field beds, with substantial variability).

What it does not reliably remove

• Heavy metals (lead, mercury, arsenic, cadmium, chromium). Some are sequestered onto fungal cell walls, but the spent substrate then becomes hazardous waste, and outflow concentrations are not reliably reduced to safe drinking-water levels.
• Pesticides, herbicides, solvents, fuels, oils — field performance is unpredictable; hydrocarbons damage or kill the bed.
• Industrial discharge of any kind — requires engineered treatment.
• Viruses — enteric viruses are 20–100 nm, far smaller than any pore in a chip-and-sand bed.
• Protozoan parasites (e.g. Giardia, Cryptosporidium) — cyst breakthrough is well-documented in slow sand filters.
• Pharmaceuticals, hormones, PFAS, nitrate.

Protecting vulnerable people

Keep mycelium-bed water and spent substrate away from the people most at risk of serious illness from contaminated water:

• Infants and young children. Never use bed water for formula, mixing food, or bathing. The bed is not a play area — fence, sign, supervise.
• Pregnant or breastfeeding women.
• Older adults and frail people.
• Anyone with a weakened immune system (HIV, chemotherapy, immunosuppressants, transplant recipients, malnutrition).
• Anyone with open wounds — bed water must not touch broken skin.

Mushrooms on the bed

Mushrooms growing on a bed that receives runoff or greywater can take up bacteria, parasites, and chemicals from that water. Wild mushrooms cannot be safely identified by appearance alone, especially in wood-chip habitats. If anyone has eaten one, contact qualified local help right away. Get help →

If you want to grow mushrooms for food, do it in a separate, clean substrate that is not part of any water-treatment system.

Handling rules

• Never drink the bed's untreated output.
• Wear gloves when handling used or spent material; wash hands afterward.
• Do not splash bed water onto your face or open wounds.
• Keep children, pets, and livestock away — fence or cover the bed.
• Post a clear sign: "Pre-treatment bed — do not drink, do not eat mushrooms here."
• Dispose of spent substrate safely — buried away from wells, streams, and food gardens, unless local guidance directs otherwise.
• Keep local emergency, clinic, and water-authority contacts written down before you build. Get help →

Site placement

• At least 30 m (about 100 ft) from any drinking-water well, spring, cistern, or surface source — further if local rules require it.
• Place beds downhill of contamination sources and downhill of drinking-water sources.
• Mark the system so it is not mistaken for a drinking-water structure.
• Account for seasonal flood lines, drought, and freezing.

When to call in expertise

• Any suspicion of industrial, mining, agricultural, or fuel contamination.
• Standing water in or near drinking sources.
• Outbreaks of waterborne illness in the community.
• Construction near regulated wetlands or protected areas.

Contact local public health workers, water utilities, NGOs, university extension services, or trained engineers.