The clearest way to cut risk on an anaerobic digestion tender is to put hard questions to the client before pricing. After a site visit to a market-waste biodigester project in southern Africa, our bid team sent eleven. This article explains the most important ones, because they apply to almost any food waste AD tender.
The challenge: a short deadline, a firm guarantee
The tender allowed just over three weeks and did not accept qualified bids. When a client rules out qualifications, every uncertainty must be priced as risk, which makes the project dearer. The bidder asked for the closing date to be moved by roughly five weeks for that reason.
The questions that matter most
1. Is the gas guarantee realistic?
The process guarantee was 450 GJ of energy a week, which implied a yield of 0.55 Nm³ of methane per kg of volatile solids, or around 87 Nm³ of methane per tonne of feed. The bidder noted that this sits at the high end of what commercial plants achieve, and that the mix of fruit and vegetables was not specified and would vary daily. Feed consistency strongly affects yield. The request was to lower the target or allow a qualified bid.
2. What contaminants arrive with the feed?
The feasibility study showed 7% paper, board, plastic, wood, metal and composites, while the site visit mentioned 4%. Because these materials are light, even a few per cent by weight is a large volume to remove. Some waste arrived in paper or plastic net bags. Questions: what exactly is in the feed, is there sand or stone, and could toxic or inhibiting substances appear?
3. Is in-digester desulphurisation required?
Injecting a little oxygen to control H₂S inside the digester is common. The bidder asked whether it was mandatory or whether post-digester gas cleaning would be acceptable.
4. Who finishes the civil works, and when?
Buildings, weighbridge, roads, fencing and paving were to be built by the client. The bidder asked whether they would be complete at handover, whether the boiler building was large enough for the macerator, motor control centre and back-up boiler, and whether the main processing building would be raised.
5. How is water managed?
The documents said both that rainwater run-off should go to the stormwater system and that rainwater tanks would be installed. Captured roof water would carry dust. Floor wash-down and leachate pit duties also needed clarifying, along with the quality required for discharge to sewer.
6. People and safety
Travel to the site raised concerns about traffic accidents and crime, which could make it hard to recruit skilled staff. The bidder asked for advice and noted that safe staff transport would add cost.
7. How are changes communicated?
Changes were to be posted on a website. The bidder asked for email notification instead, because the documents were extensive.
What to consider for your own project
- Check yield maths. Convert guarantees into yield per tonne and compare with experience.
- Characterise the feed. Contaminant content and variability set the pre-treatment size.
- Read for contradictions. Water and building sections often conflict.
- Price the people risk. Safety and access cost money.
- Protect the programme. Ask for realistic bid time.
For UK projects, see the Anaerobic Digestion Strategy and Action Plan.
FAQ
Can a bidder qualify a guarantee?
Only if the tender allows it. This one did not, which is why the question was raised.
What is volatile solids?
The organic fraction of the dry solids, which is what turns into biogas.
Why do contaminants matter?
They have to be removed before digestion and can damage pumps and mixers.
Preparing a bid?
Talk to us about reviewing your tender documents.