Water & Leachate Management

Composting is a water business wearing a carbon costume. Too much water starves the pile of oxygen; too little stops the microbes; and the water that exits — as vapor, runoff, or leachate — carries your reputation with it. This chapter is the full water story.

The pile water balance

A compost pile is a three-term water ledger: in (feedstock water, rainfall, added water) = out (evaporation, runoff, leachate, product moisture) + stored. Walk the terms:

Term	Typical magnitude	Management lever
Feedstock water	50–95% of wet feedstock mass	Recipe: dry browns absorb it; bulking agents create drainage paths
Rainfall	Variable; can exceed 1,000 L/m²/yr in wet climates	Roofs, covers, clean-water diversion — the biggest single lever in rainy regions
Evaporation from active piles	10–30 percentage points of moisture over the active phase	Turning & rewetting discipline; porosity keeps air moving
Leachate + runoff	Anything that leaves as liquid is a pollution vector	Pad design below; never "accept it, container it" without a plan
Product moisture	35–50% typical finished	Marketing + transport economics; dry before sale

The equilibrium idea A healthy pile sits at a moisture equilibrium: rewetting at turn replaces what heat evaporated; the pad sheds only what can't be reabsorbed. When rainfall exceeds evaporation (winter, wet climates), containment and covers become the whole game.

Flow diagram of the pile water balance. Water enters as feedstock moisture, rainfall, and water added at turning, and leaves as evaporation, runoff and leachate, and product moisture. The pad slopes to a sump or settling basin, and captured liquid is reused for rewetting new batches rather than discharged. — **The whole design in one picture.** Direct water in at the top and control it at the bottom: diversion keeps clean rain out of the system, and the sump is where everything that does enter gets captured. Reuse turns the sump from a liability into a dilute nutrient solution for the next batch.

Leachate chemistry: why it's not just "dirty water"

Compost leachate is a concentrated organic extract. The numbers matter for sizing treatment and for regulators:

Parameter	Typical fresh leachate range	Why it matters
Biochemical oxygen demand (BOD₅)	2,000–20,000+ mg/L	Oxygen demand in receiving water; fish-kill driver
Ammonia-N	100–2,000+ mg/L	Aquatic toxicity (un-ionized NH₃), algal growth
Total P	10–500 mg/L	Eutrophication; often the binding permit limit
Soluble salts (EC)	10–40+ dS/m	Blows up soil salinity if land-applied ungauged
Suspended solids	1,000–10,000 mg/L	Turbidity, sediment, P sorbed to particles
Heavy metals	Usually low in clean feedstocks	Concentrate in drying residue; test if biosolids inputs

The cardinal rule Never discharge leachate to a watercourse, ditch, or tile line — even 'diluted' — without a permit. It is a regulated wastewater stream wherever you are. The legal path is containment, treatment, and reuse or permitted disposal — not hope.

Pad design & containment

The pad is the water system. A capable on-farm pad (moderate scale) has:

All-weather base: compacted gravel (10–15 cm) over geotextile, or concrete for commercial yards; 2–4% slope to a collection point. The base's job is structural (equipment traffic), not watertightness by itself.
Clean-water diversion FIRST: upslope berms, roof gutters, and grade keep rain off the working area — clean water must never cross the pad. This single principle prevents 80% of leachate volume.
Containment geometry: pad is slightly crowned or sloped so all dirty water flows to a low point; a perimeter collection channel (30×30 cm min) leads to a settling basin or sump.
The sump: a simple lined catch basin (double-liner + leak detection for commercial classes) sized for your 10-yr storm + 7 days of worst-case rain, with a pump or gravity outlet — never an overflow pipe to the environment.
Curing & storage areas included: finished product sheds are the same hardstand + collection story; never let curing piles sit in a puddle.
Setbacks: to wells, watercourses, and tile inlets — commonly 30–100 m+, and more for commercial classes; verify locally (see regulatory map).

The classic failure A pad with slope but no collection is worse than a flat pad: it concentrates everything at the low corner and gives one channel to the ditch. Slope is only as good as the basin it ends in and the pump that empties it.

Treatment & reuse options

Option	How it works	Best for	Watch out
Reuse for pile rewetting	Pump sump back onto active piles (it's dilute organic fertilizer)	Small-to-medium farms; closes the loop	Don't reintroduce to near-finished product; don't oversaturate; salts concentrate with reuse
Vegetated treatment / constructed wetland	Shallow lined cells with cattails/reed canary; BOD/N removal by microbes + plants	Steady small flows, space available	Seasonal cold kills winter performance; needs outlet testing if discharging
Land application (agronomic)	Spread on bermed ground exceeding crop N/P needs, incorporated	Farms with land; the ag outlet	Cap by P, follow setbacks; many jurisdictions require a permit + testing even for this
Direct discharge	—	Never without permit	Prosecution and fish kills
Commercial treatment	Contract haul to a WWTP/lagoon	When all else fails at commercial scale	Cost per m³ is real; budget it

Reuse is the farmer's first answer — leachate is roughly a 0.1–0.5% N dilute liquid fertilizer at pile moisture, ideal for rewetting new batches and for fertigating fallow-area cover crops (sparingly, at agronomic rates). The moment you stop thinking of it as wastewater and start thinking of it as a dilute nutrient solution, the design relaxes.

Field losses: the other water story

Half of leachate risk is in the field, not the pad. Application-time losses — runoff, tile bypass, saturated-ground movement — dwarf pad leakage in most farm systems:

Incorporate within 24–48 h (or inject); surface-lying compost on sloping ground is the runoff pathway.
Respect the no-spread window: frozen, snow-covered, saturated, or standing-water fields are prohibitions, not suggestions (see timing).
Buffers: 25 m+ from streams, 10 m from tile inlets (US NRCS guidance; many states require more), grassed waterway where slope concentrates flow.
Cap the rate by P: over-application is a chronic, invisible non-point source; the rate calculator is built to catch it.
Tile-drained fields are not exempt: preferential flow can move dissolved N and salts in hours after a storm; time applications ahead of big rain events.

Regulatory baseline

Industrial stormwater (US): larger facilities (tonnage thresholds) need a permit with a written BMP plan and sampling; the plan usually includes the pad, containment, and spill-response clauses.
NPDES/state discharge: any discharge pipe to a watercourse triggers a permit; the default answer is no discharge — containment and reuse.
Agricultural exemption nuance: farm-recycling operations often sit under the solid-waste exemption but not under a water exemption — the water rules bite first (see regulatory map).
Wetland/riparian rules: setbacks from wetlands and karst (sinkholes) are strict in many regions; check locally before siting anything.
Document everything: a one-page water plan (pad layout, basins, reuse schedule, spills log) is the cheapest defense in any audit.

Testing your water: a practical panel

For most on-farm operations, an annual panel on sump water is enough (more if you discharge or land-apply):

pH, EC, BOD₅, ammonia-N, nitrate-N, total P, TSS — the core. ~$150–300 per sample; many ag labs run a standard "lagoon/manure water" panel.
Metals: only if feedstocks include biosolids or industrial byproducts.
Pathogens (fecal coliform): only if you land-apply to produce fields or discharge — and if you do either, revisit the plan.
Sample correctly: mid-depth, after mixing, in a clean 1 L bottle, on ice, ship same day.

Same water, same sampling discipline as compost: the number is only as good as the sample and the chain of custody.

Worked example: is your pad big enough?

Scale: 200 t/yr throughput, 60% moisture average, in a 900 mm/yr rainfall climate with a dry summer.

Feedstock water: 200 t × 0.60 = 120 t water in feedstocks
Reasonable active-phase evaporation: ~30% of pile water = ~36 t reabsorbed by turning/rewetting cycle
Remaining moisture leaving with product: ~25 t at 40% moisture
Balance: 120 − 36 − 25 ≈ ~60 t/yr of 'surplus' water to manage as wet-season leachate + rewetting reuse need
Rainfall on an open pad: 500 m² pad × 0.9 m = 450 t/yr — which is why roofed/curing storage and clean-water diversion cut the actual leachate volume by an order of magnitude, and why a covered receiving area is the single best water investment on a wet-climate farm.

Right-size a sump for the 10-yr storm plus a dry-week buffer, then test and reuse. The numbers above are planning guides — not design calculations; have a civil engineer check sizing at commercial scale.

Water troubleshooting

Symptom	Cause	Fix
Sump overflowing	Under-sized basin, blocked outlet, or storm coming	Overflow response plan: temporary storage, pumpdown, reduce pad area, divert clean water harder
Ponding on pad	Flat or negative slope, blocked channels, saturated base	Re-slope to collection, clear channels, add stone/reinforce base
Weeds/odor at low corner	Stagnant leachate, no circulation	Pump it out to reuse or treatment; don't let it sit
Neighbor complaint re: runoff	On-pad or field loss	Stop the source first (contain), then test, then explain with your water plan
Tile-drain discoloration downstream	Field oversaturation or preferential flow	Buffers, correct rates, avoid fall-winter applications on drained ground

Key takeaway Think of water as a third feedstock: measure what enters every pile, contain what leaves the pad, and close the loop by reusing leachate where the law and local conditions allow. Get the water right and the biology — and the regulator — are on your side.