Twenty Minutes at the Loading Arm: Throughput on a Shared Bay

Six tankers a day, one loading arm, and a facility that closes the gate at 16:30. The arithmetic says 20 minutes per truck is enough. The reality is 34, because the loading arm is shared with a sludge bay, and two of the six drivers arrive within ten minutes of each other.

Leachate solidification projects are usually designed around the batch, and the batch is usually not the constraint. The constraint is the queue.

Where the 34 minutes go

Time a load properly and it splits into five parts: positioning and hose connection, transfer, settling hold, sampling and paperwork, and disconnect. On a well-run bay those are 4, 6, 0, 5 and 3 minutes. On a typical site they are 7, 9, 20 and 5 and 4, and the twenty-minute hold is the part nobody planned for.

The hold exists because a batch needs time for the gel to finish hydrating before the load is sampled. Skip it and sampling catches a batch that is still moving, which is how a load passes on the bay and fails at the gate. Keep it and the bay becomes a bottleneck.

Move the hold off the bay

The single highest-value change is to hold in a buffer tank rather than in the truck. Batch into a 15 m3 holding tank, let it sit for the full thirty minutes while the next batch mixes, then load in six minutes with no hold at the arm. The bay time drops from 34 to 14 minutes, and the queue disappears.

This requires a tank and a transfer pump, which is capital. It also requires the holding tank to be agitated gently or not at all, because agitation after the hold re-enters air and undoes the settling.

Where capital is unavailable, sequence the day so that holds overlap with other work: load truck one, start the hold, and while it holds, weigh and stage truck two. That is scheduling rather than equipment, and it recovers about half the loss.

Sampling at the arm is the hidden cost

If every load is sampled from the truck, every load pays the hold. If sampling moves to the holding tank, the truck is only a container, and the sample represents the batch rather than the load. That is better science and better throughput at the same time, which is rare enough to be worth stating twice.

The paperwork can move too. A pre-printed ticket with batch number, density, dose and volume filled in before the truck arrives takes ninety seconds at the window instead of five minutes at the arm.

One caveat: sampling at the tank requires that the tank contents are homogeneous at the point of sampling. If the holding tank stratifies, the truck at the end of the draw gets a different material from the truck at the start, and the sample no longer represents either. Gentle recirculation for two minutes before each draw solves it, at the cost of a little air entrainment.

The shared-bay problem

Sharing a loading arm with a sludge bay means the constraint is not yours alone. Two options work. Book fixed windows and hold them, which requires the other user to accept a schedule. Or install a second, smaller arm for leachate loads, which is cheaper than it sounds because the flow rate is low and the hose run is short.

What does not work is prioritisation by arrival. It creates an incentive for drivers to arrive early, which means queuing on the access road, which creates a complaint from the neighbours and a safety issue at the gate.

A staging area for two trucks, off the public road, costs less than a second arm and removes most of the pressure. It also gives the weighbridge somewhere to put a truck that arrives while the bay is busy, which is where most of the apparent delay actually originates.

Before the next campaign, time five consecutive loads with a stopwatch and split the total into the five parts. If the hold is more than a third of the total, the answer is a buffer tank, not a faster pump. Where the batch itself is slow to wet, a grade with faster early hydration shortens the hold legitimately, and the hydration profiles show the time to 90 percent swelling for each type.