What governs your site. Project approvals, design documents, regulator requirements, SDS and manufacturer instructions, and engineering advice govern the site-specific approach. This guide is general information for planning and discussion. It does not set doses, discharge limits or compliance outcomes for any site.
Why a set-up that worked can stop working
A commissioning result describes the water and basin at one point in time. Earthworks expose new soil. Catchments are rerouted. Rain arrives. Chemical batches change. Any of these can shift how the water responds, and the dosing basis set months earlier may no longer fit.
General coagulation guidance notes that water chemistry, coagulant, dose, dispersion and pH interact, and that automation cannot repair a flawed process design [5]. A structured review is how you find which of those has moved.
Review in this order
Start with the cheapest and most fundamental checks. Many apparent chemistry problems turn out to be hydraulic or instrument problems.
- What changed: write down site activity, rainfall, new catchments, product changes and equipment changes since the last good result.
- Basin condition: sediment build-up, inlet and outlet condition, baffles, short-circuiting, and the available settling volume.
- Inflow: is the flow reaching the station within the range it was set up for?
- Water: sample and, where warranted, repeat jar testing.
- pH: measure before and after treatment and compare with earlier results.
- Dosing: re-measure delivery and check the injection and mixing point.
- Instruments: confirm calibration and look at drift [3][6].
Use the records you already have
Calibration logs, sampling results, dose adjustments and weather notes become the most valuable evidence when something changes. A review that starts from records is faster and less contentious than one that starts from opinion. If the records are thin, make improving them the first action.
Keep the approval in view
Optimisation exists to help the system do what the approval and design require. It does not change those requirements. Queensland guidance notes that larger construction sites are subject to legal requirements [1], and the procedural guide for releases to waters sets out expectations for large sites [2]. If a change in site conditions suggests the design basis no longer fits, raise it with the design engineer and, where required, the regulator.
When to involve engineering support
Bring in an RPEQ or the design engineer when a change affects the design basis, when changes to the basin arrangement are proposed, when chemistry or pH strategy changes materially, or when results remain poor after the basic checks. Critical Elixirs provides in-house RPEQ support for certification and for optimising site operations and basin performance.
Site change review checklist
- Record what changed on site and when.
- Inspect basin structure, sediment level, inlet and outlet.
- Confirm inflow is within the range the system was set up for.
- Collect a representative sample and review jar testing need.
- Measure pH before and after treatment.
- Re-measure dosing delivery and inspect the injection point.
- Check calibration status and drift on all instruments.
- Change one variable at a time and record the outcome.
- Confirm any change stays within approval conditions and design documents.
- Escalate to the design engineer if the design basis may have shifted.
Decision points
- If: A new soil type or catchment starts draining to the basin
- Take new samples and repeat jar testing before assuming the existing basis holds.
- If: Performance falls after heavy rain
- Check basin condition and sediment volume, then inflow, then dose paced to flow.
- If: Chemical supply or batch changes
- Confirm product documentation and re-check delivery and response.
- If: Basin arrangement or inlet position is altered
- Treat it as a design change and involve engineering advice before recalibrating.
Limitations and caveats
- A checklist helps structure a review. It does not replace engineering judgement for a specific site.
- This guide does not provide dose figures, discharge limits or any assurance of compliance.
- Site performance depends on design, construction, weather, operation and conditions that this guide cannot assess.
- Project approvals, design documents, regulator requirements, SDS and manufacturer instructions, and engineering advice govern the site-specific approach. This guide is general information for planning and discussion. It does not set doses, discharge limits or compliance outcomes for any site.
Sources
- [1] Queensland Government, Erosion and sediment control on construction sites (Reviewed 2 July 2026)
States that large construction sites in Queensland are subject to legal requirements, and that the procedural guides cover evolving technologies such as HES basins.
- [2] Queensland Government, Procedural guide for releases to waters from land development and construction sites 2500m² and greater (PDF)
The Queensland procedural guide for water releases from larger sites. Read it directly for the requirements that apply to your project.
- [3] Queensland Government, Quality assurance and quality control (Reviewed 7 July 2025)
Calibration processes, written procedures and record keeping are part of quality assurance. Equipment that fails calibration should not be used until the problem is resolved.
- [5] General technical reference, not Australian construction compliance guidanceIreland EPA, Optimisation of Chemical Coagulant Dosing at Water Treatment Works (2014)
Drinking-water guidance used here only for general coagulation principles: water chemistry, coagulant selection, dose and dispersion, and pH interact, and automation cannot correct a flawed process design. It is not Australian construction compliance guidance.
- [6] General technical reference, not Australian construction compliance guidanceUS EPA Region 1, Calibration of Field Instruments (Revised 2017)
General instrument practice only: calibrate before use, bracket the expected range where appropriate, keep records and check for drift. It is not Australian construction compliance guidance.
Talk to an engineer about your basin
Send location, basin arrangement, programme and approval conditions. Sampling and jar testing may be part of scoping.