Canteen Waste Solidification in Schools: Sizing Doses for 300 to 1,200 Meals a Day

School catering is not restaurant catering with smaller portions. The menu is fixed weeks ahead, the service window is twenty-five minutes long, and the waste arrives in one violent burst between 12:05 and 12:40. Whatever you dose has to work on a bin that fills in half an hour and then sits until the caretaker wheels it out.

Work back from meals to kilos to grams

Use 0.08 to 0.10 kg of plate and prep waste per meal as a planning figure for primary schools, and 0.09 to 0.12 kg for secondary schools with a wider menu and more prep trim. A 300-meal primary produces 25 to 30 kg a day; a 1,200-meal secondary produces 100 to 120 kg. That is the number you dose against, not the number of trays.

Then apply the rate. Mixed canteen waste — pasta bake, rice, vegetables, gravy, some bread — sits around 8 to 12 g/kg. Two 240-litre bins holding 55 kg each need roughly 500 to 650 grams of polymer between them. One and a half scoops per bin, if a scoop is 20 grams, is the working instruction most kitchens settle on. Where the menu includes a soup or curry day, add 3 g/kg on that day only rather than lifting the base rate for the whole week.

A 300-meal school using 28 kg a day at 10 g/kg burns 280 grams daily. Over a 190-day year that is 53 kg of polymer, which is a small line item next to the collection invoice but a real storage question.

The ten-week term problem

Schools buy in term blocks, and polymer does not care about the academic calendar. Dry SAP in a sealed bag holds its performance for two years at 15 to 25 degrees Celsius, so a 25 kg sack bought in September is still good in June. It is not good if it has been opened in a humid storeroom next to the dishwasher and left folded over. Specify a sealed container with a lid, stored off the floor, and put one named person in charge of it — usually the catering manager, not the caretaker.

Holidays create the opposite problem: bins go from full daily to empty for six weeks. Empty bins that were used for food waste should be washed and left dry, because residual gel in the base will grow mould long before term restarts.

Who actually doses it

In most school kitchens the dose is done by the kitchen porter or the catering assistant at the end of service, and that person may change every term. A laminated A4 at bin height showing one scoop per bin, a photograph of what a correctly solidified bin looks like, and a note that the scoop goes in after the food, not before, survives staff turnover better than a training session. Pair it with a simple count: scoops used per week should match bins filled per week.

Watch the drink waste. Cartons, water bottles and juice that get tipped into the food bin push the liquid load up sharply and can double the dose requirement on a Friday. Separate them at the tray-return point and the solidifier works on what it was designed for.

Limits worth stating plainly: solidifier will not make a 240-litre bin legally light enough for a single-person lift, and it does not sterilise anything. It stops the drip trail across the playground and the smell in the bin store, nothing more.

For schools running a mixed menu across several sites, the high-absorption SAP grades are worth comparing against your soup and stew days before you fix a contract quantity.