In my last article, I have outlined why mono-incineration plants for sewage sludge will grow in importance. I want to explain this in more detail. Sewage sludge has to be dried before it is being used in mono incineration plants, and although it has a very low calorific value, it is possible to generate electricity from the incineration process which can be fed into the electricity grid. Another major advantage of sewage sludge is the production of biogas and methane, which can be used in industrial plants nearby.
One of the major disadvantages is that sewage sludge plants are often found near major urban agglomerations, whereas in agricultural areas, there is still significant potential to exploit sewage sludge, but the very fact that the sewage plants are at quite a distance to urban centers means that it is more costly to transport the material to the mono-incineration plants. With growing distance, it becomes more and more unattractive to deliver the material to mono-incineration plants, where sewage sludge can be used to produce energy, and to remove phosphorus from the input material.
In some countries, Austria is one good example that comes to my mind, it is not very common to deliver sewage to sewage plants, simply because the pipes only exist in urban agglomerations. That also means the sewage has to be stored at storage sites, and is collected by municipalities that transport the sewage material to treatment facilities. Due to transportation issues it is generally preferred to transport dried residuals, which eliminates some of the toxins.
As we now know, toxins are a major issue limiting the use of sewage sludge in agriculture. It has been found that the uranium content, for example, contained within the sewage sludge has risen over time due to the fact that certain layers of rock in Western Sahara contain significant amounts of uranium. That is partly the reason why the German government has ruled that sewage sludge can no longer be used as fertilizer in agriculture, even though it also contains significant amounts of nitrogen. Just as phosphorus, nitrogen is a natural fertilizer for plants. As the soil depletes of nutrients over time, as has happened in the United States for the past 100 years, especially in the American Midwest, more fertilizer is needed to keep up agricultural production. It is fairly easy to produce nitrogen, but extremely difficult for producers to find more phosphorus, which cannot be produced synthetically at an industrial scale like nitrogen.
The stakes are high; and mono-incineration plants help increase energy production from environmentally-friendly sources, while at the same time they get rid of waste, and produce phosphorus rock as a suitable alternative to mining. It helps reduce one’s dependence on one sole supplier supplying this commodity globally. But it will not help the long-term decline of phosphorus found on fields, because phosphorus is washed out and fed into rivers, which deposit phosphorus in the oceans. The phosphorus cycle means that phosphorus is used up much much faster than it can be replenished by us. In essence, mining non-detrital sedimentary rock will not solve the problem of replenishing phosphorus, nor will the fact that we incinerate dried sewage sludge to obtain phosphorus, and produce energy while we are doing it.
One can easily see that there is great potential to use mono-incineration plants to burn sewage sludge world-wide. There is enormous potential in the U.S., which has been pointed out by Jordan Peccia and Paul Westerhoff in their paper on “We Should Expect More Out of Our Sewage Sludge” which was published by Environmental Science & Technology in 2015. In the U.S. alone, 7.2 dry tons of sewage sludge are produced every year. A lot of it is being wasted, the phosphorus is washed out from the soil. In the United States, only 15% of all sludge from sewage plants are being incinerated. What makes matters even worse is that the United States first uses up its domestic supplies of phosphorus, making the U.S. more dependant on imports, and in the period from 2013 to 2016 imports from Morocco alone accounted for 32% according to the U.S. Geological Survey. Whereas the U.S. produced 27,700 tonnes of phosphorus rock in 2017, Morocco produced 27,000 tons, which is roughly the same amount. The real differences are staggering: United States reserves stood at 1 million tons, while Morocco’s reserves were 50 million tons! The Moroccan figures obviously include Western Sahara.
Germany and Austria also incinerate roughly 15% of their sewage sludge, similar to the U.S., but Germany is expected to increase that ratio significantly due to a new regulatory measure in place which says that by 2029 no sludge will be allowed to be used in agriculture. This was detailed in a research study by the German government.
But the benefits of treating sewage sludge through incineration go much further, because the sewage sludge ash (SSA) produced from the incineration process can be incorporated in cementitious materials such as Portland cement, as has been pointed out by D. Vouk, D. Nakic, N. Stirmer and C.R. Cheeseman. Sewage sludge ash is particularly suitable for Portland cement admixture and to use can be used as a substitute for other materials. Sewage sludge ash will become more widely available in the European Union, as more sewage sludge is thermally treated.