An Asset, Not a Waste: What Can Be Found in Wastewater

wastewater water treatment

As we lead our lives, we barely ever think of how much water we need for it. Cooking, household maintenance, shower, toilet, laundry — this sums up to about 150 to 400 litres per capita daily. Most of this amount will eventually become wastewater that must be treated before it can be discharged into nature. Water treatment has never been cheap — in the US the water treatment expenses make up a third of the municipal energy budgets.

What’s there to look for?

Water treatment specialists know that wastewater is actually an asset rather than a waste. In fact, wastewater can contain three times the amount of energy needed to treat it. Apart from that, a number of valuable elements can be found in wastewater, and those may be easier to extract from there rather than abuse natural resources. Here are a few examples of what wastewater is rich in.

Methane

Illinois-based researcher Yalin Li has been exploring ways to produce energy from wastewater since 2017. One of his technologies suggests using microorganisms to extract energy from household waste. The mechanism is as follows: water tanks are filled with wastewater where the favorable conditions for bacteria are created. During a process called anaerobic digestion the bacteria decompose organic waste into water, methane, and carbon dioxide. Methane is later used as a fuel for furnaces or water heaters, and the leftover organic waste can be processed into fertilizers.

water treatment fermentation
Inside these tanks methane is formed via anaerobic digestion of wastewater. Credit: Frank Hebbert/Wikimedia

Biofuel

Another area of research Li takes interest in is the use of micro-algae in water treatment. Some species can be grown directly in wastewater, where the organic waste can serve as a perfect nutrient for them. The micro-algae grown this way can later become biofuel.

[…] Many researchers – including me – are very interested in microalgae. Microalgae are promising feedstocks for biofuels, and some of them can grow in wastewater. My colleagues and I have designed hydrothermal systems to turn wastewater-grown microalgae into biofuels. They are still being tested in the lab, but we hope to scale them up in the near future. – Yalin Li

Ammonia

Wastewater is rich in ammonia, with the majority of it coming from urine from toilets. Ammonia, or rather ammonia-based fertilizers are widely used both on large fields and on small private crops. Today’s water treatment technology allows the extraction of ammonia directly from the sewage.

Apart from ammonia, significant amounts of phosphorus and potassium can be found in organic waste, too. These are another two elements plants need for fast growth. Rich Earth Institute, a Vermont-based non-profit organization, is exploring ways to produce fertilizers from urine.

Clearly, it would be much more efficient to directly extract the ammonia from wastewater without converting it. One way is to use urine-diverting toilets, which already are commercially available, to separate urine from other sources of wastewater. Then the collected urine could be used as fertilizer after sanitizing it to remove pathogens. — Yalin Li

Sanitized urine also contains other nutrients like phosphorus and potassium. The Rich Earth Institute is researching ways to turn human urine into fertilizer. The institute is testing harvested urine on real crops, and has found that it works effectively.

Hard and precious metals

While searching for ammonia, methane, and phosphorus, one can find gold. Valued up to millions US dollars, particles of gold and other precious metals flow in wastewater and there is a way to extract them all from the solid matter of wastewaters. Because heavy metals like mercury or lead pose a threat to aquatic life, they need to be removed before the water is dumped back into nature.

Researchers are developing new ways to remove and reuse these metals. These techniques are at a very early stage and it is not clear yet whether they will make sense economy-wise, but they have the potential to make wastewater even more valuable.

Just a beginning

Like other researchers, Yalin Li believes that further development of water treatment technology will eventually allow us to extract even more use and profit from wastewater. The only difficulty is that the technologies described above are still at a very early stage. It may take years and dozens of years before we begin using them on an industrial scale. But as long as they are in development and showing good potential, there is hope that they will gain necessary attention and be of use in water treatment.

To me, this is just a beginning. With proper techniques, “wastewater” can offer us much more – and I very much look forward to the day when there is no “wastewater,” just “used water.” — Yalin Li