Primary Sedimentation: From Neglected Common Asset to Future Keystone Technology

Palmer, S. MWH (UK) Ltd


Primary sedimentation is one of the first widely adapted wastewater treatment technologies and is now arguably one of the most underrated and unappreciated. With wastewater treatment operating costs set to rise over the next few decades and sewage sludge offering wastewater treatment facilities their principal source of indigenous renewable energy, primary treatment now offers opportunities to increase energy recovery from conventional digestion via a low operating carbon footprint technology.

This paper provides a brief review of primary sedimentation operations and the science and current understanding of primary sedimentation and identifies the remaining gap in that understanding, which means that even now with CFD modelling, conventional primary sedimentation performance cannot be predicted.  The paper describes how to fill that information gap and how that information can be used to further develop existing primary sedimentation to maximise its solids capture and BOD removal. The effect of tank operation on tank BOD and solids capture and potential nuisance microbiology is described, including odour production and a description of best operational practice to achieve optimum performance with conventional tanks.

Primary sedimentation emerges from this review as a carbon efficient, energy recovery enabling technology that could be readily upgraded to maximise these attributes to meet emerging challenges to municipal wastewater treatment.


Primary tanks, upgrading primary treatment, steady state one dimensional modelling, optimising energy recovery, carbon footprint reduction, lowering operating costs.


The water industry is now being confronted by new risk profile over the next 40 years which is entirely different to that the industry has experienced over the past 40 (with the exception of some investment projects provoked by energy prices spikes in the oil crises of 1975 and the mid-1980s).

This new risk profile includes:

  • Increased operating costs; arising principally from energy costs which also feed through into increased fuel, electricity and chemical costs,
  • Climate change and carbon, driving a need to invest in carbon reduction (1).

The future technology profile that can mitigate these risks therefore requires technologies that are very carbon efficient (meaning they treat wastewater efficiently while having low consequent greenhouse gas emissions). One aspect of carbon efficiency is electricity demand: in the UK some 70% of carbon footprint in wastewater treatment operations on larger plants (OFWAT Category 6) typically arises from national grid electricity consumption.  Grid electricity Greenhouse gas emissions are termed Scope 2 emissions in carbon accounting.

Reduction of operational carbon footprint therefore has reductions in grid supplied electricity consumption as a major aim. This is reinforced by another driver- the limits on energy efficiency interventions. To date, best practice energy efficiency measures implemented in modern conventional municipal wastewater treatment are typically achieving electricity demand reductions of 20-30%. This means that some 70-80% of grid electricity demand remains after implementing energy efficiency. To reduce Scope 2 carbon emissions to zero- in effect, energy neutrality, in electricity terms, the sewage works should replace grid electricity with renewable electricity.

This would eliminate the works electricity operating cost, neatly converging operating cost reduction and carbon footprint reduction.

A technology that is inherently low carbon due to low initial power consumption, which can also offer increased renewable energy recovery potential, would ideally fit the new operating risk profile for wastewater treatment.

One such technology is primary treatment in the form of primary sedimentation, for the following reasons.

Firstly, primary sedimentation settles particles under gravity in a tank. Energy inputs on large tanks are limited to scraper bridge drives which move sludge to the tank sludge collection hopper and the sludge removal pumps which desludge the sludge hoppers.  Secondly, primary sludge itself has the highest biogas potential of the sludge types, being composed of raw sludge which has not yet undergone any biological degradation thus reducing its energy content. Consequently, primary sludge has the potential to produce 149% more biogas than biological waste sludge in conventional sewage works anaerobic digestion and increasing the capture of primary sludge in primary sedimentation and hence the ratio of primary sludge to secondary biological sludge, can significantly increase biogas production. Finally, the larger fraction of municipal wastewater BOD (55%) is typically particulate, therefore boosting primary sludge capture also reduces downstream BOD load. The benefits of this BOD load reduction are increased downstream biological treatment capacity and where that capacity is provided by an activated sludge plant, slight reduction in aeration costs and associated Scope 2 carbon emissions.

On small tanks (for small sewage works), vertical flow tanks of the pyramidal type (Figure 1), which have a square plan layout, do not even have scrapers and are often desludged hydrostatically rather than by pump. Such a tank has no drive units, no electricity consumption and consequently, produces no Scope 2 carbon emissions.

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