A treatment plant’s electricity cost is set by a handful of decisions, not by the continuous hum of the equipment: when the pumps run, when the blowers turn down, when the battery charges or discharges, and whether the biogas engine runs or idles. Price-responsive control automates those decisions against the market, the tariff, and the site’s own physical limits, every five minutes, without an operator having to watch the price screen.

The three price signals, and one cost

Every plant sits under at least one, and usually several, sources of price signal at once:

• The wholesale spot price — a new price every five minutes, reflecting supply and demand across the grid. (See The NEM Explained.)
• The retail tariff — time-of-use rates that vary by peak, shoulder, and off-peak periods, and across the year. (See Electricity Tariffs Explained.)
• The maximum demand charge — where a small number of peak intervals each month set the network demand charge for the whole billing period, the most expensive intervals are the ones the scheme most wants to avoid.

The control scheme reads whichever of these apply to the site, combines them into a single effective cost signal, and weighs that signal against what the plant can safely do at that moment. Which signal dominates depends on the site’s commercial arrangement — a plant on the spot market responds to the five minute price, one on a retail tariff responds to the time-of-use calendar, and both must respect the demand charge.

The plant as a storage battery

Flexible load exists because treatment processes hold inventory. An inlet balancing tank or wet well holds a volume of incoming flow that can be pumped now or pumped later, within its operating range. That volume is the plant’s energy storage: pump when electricity is cheap and the tank drains, hold back when it is expensive and the tank fills. The higher the stored volume and the wider the operating band between the low and high working levels, the more load can be shifted in time.

The controller converts the current price into a target operating level for that tank. Cheap electricity lowers the target, so the pumps run harder and draw the tank down. Expensive electricity raises it, so the pumps ease back and the tank stores the incoming flow. The response is smooth and continuous rather than a hard on-off switch, so the plant gradually tilts toward cheap windows and away from expensive ones, without sudden switching and without running the tank dry or overflowing it.

Looking ahead, not just reacting

Reacting to the price at this instant is not enough, because the plant has inertia: it takes time to move water, to charge a battery, or to warm a biogas engine to the point it can run. The scheme therefore looks ahead, using forecasts of price, solar, weather, and incoming flow, and positions the plant in advance.

• An hour ahead, the scheme holds the plant ready for the next high price interval, and pre-empts storm inflows by drawing the tank down in advance.
• A day or more ahead, it positions the reservoir and battery to ride out known peaks and to make best use of tomorrow’s forecast solar and price profile.
• On a five minute basis, it dispatches pumps and battery against the live price, exactly as it will in operation.

Where the price is never allowed to win

The commercial objective is always subordinate to the process. The scheme carries the site’s real physical constraints with it at every step and enforces them automatically:

• Reservoir and wet well limits — hard low and high levels that are never breached, however extreme the price.
• Storm handling — when large inflows are coming, the scheme drains the tank early so capacity is available, treating flood risk as the highest priority signal of all.
• Hydraulics and pump behaviour — the real pipework, friction, and pump curves determine what flow is actually achievable, so the scheme never asks for something the pipework cannot deliver.
• Pump wear — start and stop counts are damped so the scheme does not save pennies on electricity and spend dollars on switchgear and impeller wear.
• A maximum import cap — where a site wants to cap how much power it draws from the grid, the scheme sheds load to respect the cap while never endangering the process.
• Operator override — operators retain full manual control at all times.

Solar and battery as part of the same decision

Where a site has on-site solar and battery storage, they join the same coordinated decision rather than being controlled separately. Solar generation offsets imported power whenever the sun is shining. The battery charges when electricity is cheap or free — off peak, at low spot prices, or from solar surplus — and discharges when prices are high, when a peak interval is coming, or when it can protect the site’s maximum demand. The pumps, the battery, and the solar are dispatched against the same cost signal and the same physical constraints, so one does not undo the work of another.

Designed by simulation, not by guesswork

The logic inside the controller is not set by rule of thumb. Before anything runs live, the site is built into a simulation model, validated against actual plant data, and then run over a full year of real and forecast prices, weather, and flows. The control settings — how aggressively the scheme responds to price, how it balances stored volume against cost, and how it trades pump starts against energy savings — are tuned against that simulated year so the outcome is minimised for the whole billing period, not optimised to look good in any single interval. The same engine that designs the scheme is the one that can replay and verify it, and the same model becomes the ongoing reference for monitoring.

That is the point of Integrated Industrial Energy Control at Delprosa: the control logic that runs live is the same logic that was validated in simulation, so a client receives a quantified forecast of the outcome — the bill, the demand peaks, the pump starts, the overflow risk — before committing to anything. The process from assessment to live operation is set out in How We Work, and the three commercial routes into it are described on Energy Management.

Looking forward: blowers and biogas cogeneration in the same scheme

The same control philosophy extends naturally to the two other large loads and the plant’s own generation. Work is underway to bring them into the same coordinated decision, so a plant responds as one integrated system rather than a set of separately scheduled assets.

With all of the assets in one scheme, the plant dispatches against a single cost signal and a single set of process constraints: solar and stored energy first when it is cheapest, biogas generation next when the price justifies it, grid import to cover the rest, and every asset held within its operating envelope. The simulation model extends to cover the digester gas balance, the engine’s electrical and heat output, and the blower and dissolved oxygen response, so the same design, tuning, and verification process that applies to the pumps today applies to the whole plant tomorrow.

What this means for your site

A plant under price-responsive control does not need someone watching the market to run it profitably. The decisions are made automatically, within limits that are set and enforced by the process itself, and the expected outcome — a lower bill, fewer peak intervals, no lost effluent quality — is quantified in advance against your actual tariff and market exposure. Whether a site is best served by wholesale market flexibility, tariff optimisation, or evaluating a new retail offer depends on its circumstances; the control method underneath is the same.

You can see the method in action on a full year replay, with a plant responding to real wholesale prices across four scenarios, on the sample simulation page.