Cleaning water after use

Sewers, treatment, sludge, biogas, nutrients and reclaimed water. Theory, formulas, worked examples and applications.

Cleaning water after use

The sewer does not always carry only what is flushed at home

The sewer system carries already-used water from homes, businesses and industries toward the treatment plant, but in many cities with "combined" networks — the oldest ones, which combine wastewater and rainwater into a single system — it also collects the water falling during a storm. When an intense storm exceeds that combined network's transport or treatment capacity, the system may be forced to trigger "overflows": direct discharges of a mix of rainwater and incompletely treated wastewater, straight into a river or the sea, precisely to prevent that mix from flooding streets and homes. It is a design trade-off, not an isolated failure: combined networks existed before this problem was well understood, and modernizing them to separate both flows is considerable civil engineering work many cities are still completing.

Inside the treatment plant: physics, biology and, depending on the case, additional chemistry

At the treatment plant, screens and grit chambers first remove the largest objects and sand or gravel carried by the water. Then, sedimentation tanks separate by gravity much of the remaining suspended solids. The biological heart of the process is populations of microorganisms that consume the organic matter dissolved in the water as part of their own metabolism, drastically reducing its polluting load. Depending on applicable regulation and the sensitivity of the receiving environment, specific additional treatments are added to remove nitrogen, phosphorus, or emerging microcontaminants such as pharmaceutical residues or personal care products — treatments not every treatment plant in the world includes yet, precisely because they are more recent and more costly than conventional biological treatment.

The energy cost almost no one sees, and the one almost no one captures

Aerating the water so microorganisms have the oxygen they need consumes a significant amount of electricity — by far the largest single energy consumer within a conventional treatment plant. Lift pumps, chemical reagent dosing, filters and final disinfection systems each add their own electricity consumption. But the process can also generate its own energy resource: the organic matter concentrated in residual sludge can be treated in anaerobic digesters that produce biogas, a fuel many modern plants use to generate part of their own electricity or heat, recovering a fraction of the energy invested in the whole process. Recovered phosphorus, already-treated water, and the process's own waste heat can become usable resources, provided contaminants have been properly controlled before attempting to recover them.

Sludge concentrates what the water no longer carries

The residual sludge left after treatment concentrates much of what has been removed from the water, including trace heavy metals and a significant fraction of the microplastics that arrived with the wastewater. Applying that sludge to agricultural soil as fertilizer, thermally treating it, or incinerating it are the three main management routes, and each requires strict regulatory limits and continuous monitoring of its exact composition. The more water is polluted at the source — uncontrolled industrial discharges, household chemicals flushed down the drain —, the harder and more expensive it becomes to safely close this whole cycle.

Reusing at exactly the quality each destination requires, no more and no less

Already-treated water can be reused for agricultural irrigation, urban cleaning, certain industrial processes, controlled aquifer recharge, or — with the strictest and least common sanitary barriers — indirect human consumption. Each of those destinations requires a different level of treatment, and it makes neither economic nor energy sense to purify every liter of wastewater to a standard the final use does not require at all. Nor is it acceptable to reuse water without the sanitary barriers appropriate to the chosen destination, even if that means a higher treatment cost. The most sensible sequence, in almost any context, is: prevent polluting discharges at the source, reduce water consumption in the first place, treat with demonstrated reliability, recover the resources — biogas, phosphorus, heat — that are reasonable to recover, and choose for each final destination exactly the quality level that specific use requires.

Reuse at the right quality, no more and no less

In favor of recovering resources: biogas from sludge, recovered phosphorus and already-treated water can become real resources, reducing the net energy cost of the whole cycle.

To watch: aerating water for biological treatment is, by far, a conventional treatment plant's largest electricity consumer; and sludge concentrates metals and microplastics that require strict management and regulatory limits.

Do not purify what does not need it. Irrigation, urban cleaning, industry, aquifer recharge or consumption require very different treatment levels — treating every liter to the highest possible standard wastes energy with no additional health benefit.

The most sensible sequence: prevent discharges at the source, reduce consumption, treat with demonstrated reliability, recover reasonable resources, and choose for each destination exactly the quality that specific use requires.

The numbers, with their source

Between 0.5 and 1.5 kWh for every cubic meter treated

A conventional municipal treatment plant's electricity consumption typically sits between 0.5 and 1.5 kWh per cubic meter of wastewater treated, though detailed studies find somewhat wider ranges, from 0.13 to 0.79 kWh/m³ in some specific configurations, depending on technology, plant design and the quality required of the treated water. Aeration — injecting air so biological-treatment microorganisms have oxygen — alone represents more than 60% of all the energy consumed in a conventional treatment plant, by far the largest single consumer in the whole process.

Sources: MDPI — Energy Consumption in Water/Wastewater Treatment Industry; Water & Wastewater — How Much Electricity Does A Wastewater Treatment Plant Use.

Up to 40% of a small municipality's electricity bill

In small communities, the municipal treatment plant can represent between 25% and 40% of the entire municipal electricity budget — a surprisingly high figure for a single piece of infrastructure. Globally, the world's wastewater treatment plants together consume approximately 3% to 4% of all electricity generated on the planet, while in the United States specifically it represents around 0.8% of the country's total electricity consumption.

Sources: MDPI — Energy Consumption in Water/Wastewater Treatment Industry; Science the Earth — Electricity Use and Management in the Municipal Water Supply and Wastewater Industries.

What this table is missing

These figures describe electricity consumption; they do not include how much energy is actually recovered as biogas at plants that do have anaerobic sludge digestion — a figure that varies enormously by the size and design of each specific facility, and that can offset a relevant part, though almost never all, of the plant's own electricity consumption.

Sources