Composting and Organic Waste Facility Leachate Treatment: Ammonia and High BOD

Leachate from a composting site is not landfill leachate, and treating it as such is an expensive mistake. It comes from fresh, rapidly degrading organic matter rather than from a decade-old refuse mass, which means the BOD to COD ratio is high, the material is genuinely biodegradable, and the ammonia load arrives all at once. It also arrives in the first two weeks of each batch and then largely stops, which does more to shape the treatment plant than the chemistry does.

How Much, and When

Generation rate depends on feedstock and on how wet the incoming material is. Food waste and mixed organics yield roughly 100 to 300 litres of leachate per tonne processed; green waste and woody feedstocks far less, and dry feedstocks may generate none at all unless the yard gets heavy rain.

Timing is sharply peaked. Fresh material drains within the first days as cell water is released, and the strongest leachate — COD of 30,000 to 70,000 mg/L, BOD of 20,000 to 40,000 mg/L — appears in that window. By the time the windrow is four weeks old, generation has dropped substantially and strength with it. Plants dealing with comparable loads face many of the same trade-offs covered in landfill leachate membrane concentration and evaporation.

A treatment plant sized for the peak sits idle most of the year. One sized for the average is overwhelmed in week one. Storage with equalisation, sized on the peak week rather than the mean, is what reconciles the two, and it is the item most often value-engineered out and then retrofitted.

Recirculation: Free Treatment With a Ceiling

Most sites recirculate leachate back over the windrows or into the in-vessel reactor. It disposes of the water by evaporation, keeps the moisture content in the range the microbiology wants, and returns nutrients to the compost rather than sending them to a treatment plant. It is the correct first step almost everywhere.

The ceiling is water balance and oxygen. Recirculate more than the pile can evaporate or hold and it goes anaerobic, producing volatile fatty acids, hydrogen sulphide and the smell that gets a site shut down. Over-wet compost also fails to reach temperature, which undermines pathogen kill and the composting process itself. The process logic sits close to what is described in BOD Removal, pH Neutralization and Biogas Recovery, particularly on the pretreatment side.

In practice, recirculation works while the weather and the feedstock cooperate, and stops working in a wet winter or with a wet feedstock. The treatment plant exists for those periods. Sizing it on the assumption that recirculation always works is how sites end up with an overflow. Much of this mirrors the situation set out in Pesticide and Herbicide Manufacturing Wastewater Treatment.

Ammonia Is the Constituent That Breaks Plants

Protein degradation releases ammonia, and compost leachate commonly carries 500 to 3,000 mg/L of ammoniacal nitrogen. That is high enough to inhibit nitrifying bacteria directly, and free ammonia inhibition becomes significant as pH and temperature rise, which they do in a composting environment.

The biology still works, but it needs design that accounts for it: a long solids retention time so the slow-growing nitrifiers are not washed out, generous aeration capacity, sufficient alkalinity because nitrification consumes roughly 7 mg of alkalinity as CaCO3 per mg of ammonia oxidised, and usually a dilution stream to keep the influent concentration down.

Denitrification needs carbon, and by the time the leachate has been through an aerobic stage the readily degradable carbon is largely gone. Plants that successfully hit low total nitrogen usually either add an external carbon source for the anoxic zone or divert raw leachate into the anoxic zone as the carbon supply — the second is cheaper and needs flow control to work reliably.

What This Looks Like in Practice

For comparison, our 2022 project in Chengdu, Sichuan ran at 76 m³/day with influent COD near 2,000 mg/L and treated water around 300 mg/L. That is 85% removal in practice, not on paper.

Figures are site-measured values from commissioned plants, not modelled estimates.

Anaerobic digestion of the strong fraction is a good match where the site already has a digester, and co-digestion with food waste is increasingly the reason composting and AD sit on the same site. Where there is no digester, aerobic treatment of the whole flow is simpler: sequencing batch reactors, extended aeration or MBR, with nitrification-denitrification and chemical phosphorus removal.

Constructed wetlands and reed beds are used on smaller sites and work well on the dilute fraction after solids removal, particularly as a polishing stage. They need land, they slow down in winter, and they do not remove ammonia well without a nitrifying stage ahead of them. We have written about the downstream implications separately in Sludge Separation, Coagulation, Flocculation and Filtration.

Discharge to sewer under a trade waste agreement is the pragmatic answer for many facilities and is often the cheapest compliant route, provided the receiving works has capacity and the ammonia load is acceptable to it. Evaporation to dryness is the last resort and is energy-prohibitive except on small flows.

Yard Runoff and Odour

Any outdoor composting pad produces contaminated runoff whenever it rains, and the first flush after a dry period carries the highest load. Collection with a separate first-flush tank, plus clean water diversion around the pad, keeps this from becoming a much larger volume of dilute leachate.

Leachate storage is a significant odour source — an open tank of high-BOD liquid goes septic within days. Covered tanks, vent gas treatment through a biofilter or the compost pile itself, and short storage times all help, and all three are cheaper than dealing with a complaint.

Combining Treatment Stages

The same selection criteria reappear in Aluminum Anodizing Wastewater Treatment system, which is worth opening alongside this one. Most facilities do not run a single clean stream, and a shared equalisation and biological stage is usually the economical answer once the streams are chemically compatible.

Why Choose Baihuipu as Your Manufacturer

Two decades in this industry teaches you that most treatment problems are procurement problems in disguise. Someone bought the cheapest skid, nobody checked the equalisation volume, and eighteen months on the plant cannot hold its consent limit. We build differently.

In-House Fabrication

Our Guangdong works produces everything from standard modular units to fully bespoke lines. Building in-house keeps cost, schedule and quality under our own control rather than a subcontractor's goodwill.

20+ Years Across Dozens of Industries

Experience matters most when your waste stream behaves nothing like the textbook example. We have dealt with toxic organics, heavy metals, extreme salinity and refractory COD in food processing, chemical manufacturing, electroplating, mining and municipal projects.

Twenty-Plus Export Markets

We have shipped to the United States, Canada, Russia, Indonesia, India, Nigeria and Vietnam among others. Crating, export documentation and certificates are handled in-house, which keeps customs surprises to a minimum.

Complete System Responsibility

One supplier, one process guarantee. We take the plant from preliminary screening through to membrane separation, evaporation and brine management, with process, mechanical, electrical and controls engineering handled internally.

Spares, Service and Commissioning

Membranes, dosing pumps, diffusers, instrumentation and blowers are stocked for the systems we sell. Our control systems support remote diagnostics, and we can put an engineer on site for commissioning, training or an unplanned shutdown.

Documentation You Can Actually Audit

Each system ships with as-built drawings, material certificates, welding records and a functional specification. If a client's own consultant wants to review the process calculation basis, we hand it over rather than treating it as proprietary.

Getting Started

Send us a water analysis and your production profile and we will tell you honestly what the process should look like and roughly what it costs to build and run. Reach our engineering team on WhatsApp: +86 13631765076 or via hkbhp.com.

WhatsApp: +86 13631765076

Frequently Asked Questions

What does it cost to run, per cubic metre?

Energy dominates, then chemicals, then sludge disposal. We give you a per-cubic-metre breakdown during proposal so you can compare it against your sewer charge — treating too aggressively is a common and expensive mistake.

What information do you need to quote?

Water analysis for your worst-case stream, peak and average flows, discharge limits, available footprint and utilities. If you do not have a full analysis yet, we can start from a partial one and flag the gaps.

Can the plant be modified later if our production changes?

Yes, if you tell us about likely expansion up front. Leaving room in the civil works and oversizing the equalisation tank costs far less than retrofitting later. Composting and Organic Waste Facility Leachate Treatment rarely fails because of bad chemistry — it usually fails because nobody planned for the second production line.

Can you work with our existing equipment?

Often yes. We regularly integrate into existing civil structures where the concrete is sound, replacing internals and controls rather than starting over. We will tell you honestly when legacy kit is beyond economic repair.