Fourteen Weeks Without a Sewer: Gel Deliveries to a Tower Crane Site

The site compound sits on a demolition plot in a city centre, and there is no sewer connection until the basement slab is poured. Fourteen weeks, 120 workers, 40,000 square metres of floor plate under construction, and eleven portable units along the hoarding. The site manager’s question at the first meeting was blunt: what does the powder actually cost me over the whole job, and how often does a lorry have to come through the gate?

That is the right question, because on a construction site the constraint is rarely the powder. It is the delivery window, the hoarding gate, and the fact that a tanker blocking a concrete pour costs more per hour than a month of gel.

The site brief, in numbers

Eleven units, mostly 900-litre, serviced twice a week by a vacuum tanker because the site cannot take a conventional pump-out vehicle through the hoarding after 07:30. A workforce of 120, of whom about 90 are on site on any given day, with an average of two visits per worker per shift. That is roughly 180 uses per day across eleven units, or 16 uses per unit per day, which sounds low until you account for the fact that four units sit next to the canteen and take most of the traffic.

Weekly liquid load: about 1,260 uses at 300 ml is 378 litres of urine, plus hand-wash water and cleaning water, call it 600 litres a week, plus the recharge volume the service crew adds. Over fourteen weeks that is 8,400 litres, or roughly 9 tonnes of liquid, none of which can go to a drain.

Consumption over the whole job

At 1.6 g per litre of total liquid and a service that recharges to 150 litres per unit, each unit needs about 240 g per service, so 11 units twice a week is 5.3 kg per week. Fourteen weeks is 74 kg, or three 25 kg sacks, plus a margin for the hot weeks in July when consumption rises because workers drink more and the units get more use.

The number that surprises site managers is the tanker column. Without gel, the same eleven units on the same workforce would need a third weekly visit in summer, because a unit at 85 per cent full is unusable and a site will not accept a closed toilet at 10:00. Three extra tanker visits over fourteen weeks, at a typical callout rate, is more than the entire powder bill.

Weather, dust and the delivery schedule

Construction sites are dirty in a specific way. Dust settles on the units and gets into the bowl, and a gel that forms around a layer of cement dust makes a mass with a gritty crust that resists the pump-out hose. The practical fix is a rinse before dosing, not more powder — a two-litre rinse down the bowl before the recharge keeps the dust from becoming part of the gel matrix.

Rain is the other variable. Units standing on bare ground take surface water through the door threshold, and a summer thunderstorm can add 50 litres to a unit in an hour. The site above had two units in a dip and lost a week of gel performance to it, until the units were relocated onto a timber platform 100 mm high. That is a €60 fix that outperformed every dosing change we discussed.

What to put in the site waste plan

Three lines. Dose rate in grams per litre of total liquid, with the recharge volume stated. Maximum interval between services, stated in days not visits, because a bank holiday Monday changes the pattern. And the contingency for a missed collection: a spare 5 kg of powder on site and a named person authorised to add a top-up dose.

The last one matters more than it looks. On a site with a fixed tanker window, a missed visit is normal, and a unit that is 90 per cent full on Friday will be over the seat line by Monday. A 300 g top-up dose on the Friday, added by the site labourer with a scoop and a clipboard, buys two days. If you are specifying for a multi-phase project, the SAP powder product range shows which grades tolerate a partial refill without turning into a crust, and that property is worth more on a construction site than a headline absorption ratio.