To sludge or not to sludge, that's the question

Puck Norell

23rd July 2018

In this adventure, the human practices team and Andreas went to Henriksdals wastewater treatment plant to learn more about the workflow of treatment plants, and to get some professional insight to our project. We learned a lot, not only how plants are built up and how we can develop our project around them, but also about how important sludge is. What role does sludge play in the plants, where does it end up eventually, and what does it contain? Jump on the bus together with us to get to know more!

It was to our disappointment that the bus going towards Henriksdals wastewater treatment plant did not have an air conditioner. The 8 minutes we travelled from Slussen towards our final destination felt like a lifetime, but we were soon off the bus and ready to absorb some new knowledge. We were going to Henriksdal to get some inspiration and professional input on our idea, and to check out how the plant was operated.

We were welcomed at the entrance with a stale smell of wastewater, but our spirits were not to be budged. Our contact, Sofia Andersson met us in a seminar room upstairs, and we went directly down to business. Sofia was the perfect person to discuss with thanks to her extensive knowledge within the field. She explained each step of the plant flow, and we got to ask questions along the way.

There are two important factors within the work in wastewater treatment plant. The first is the primary treatment. This treatment has the purpose of protecting the instruments in the following treatment steps, examples of these are pumps and stirrers. In order to protect, the first step is a grid which sorts out things that should not have been flushed down the toilet in the first place. Remember, that the only things that should be flushed down are your body contents and toilet paper! The second thing is the use of multiple steps to ensure correct treatment of water. One of the first steps within the work is using sand particles which filter the water. In this method, oxygen is used to make organic material remain in the liquid while sand is removed. The sand is later cleaned and used for construction, while the material that is filtered out is rinsed, packed and burned with household waste by Fortum at another facility.

A lot of phosphorus compounds can be found in the wastewater that arrives to the plant, and in order to remove these, ferrous sulphate is used to precipitate the compounds. Wastewater plants in Sweden today are regulated to measure and minimize the levels of phosphorus,nitrogen and organic materia. Important to know is that Switzerland is the only country at this point that regulates measuring and limiting pharmaceuticals in the incoming water.

Nitrogen that comes in to Henriksdals wastewater treatment plant is removed biologically by a combination of nitrifying and denitrifying bacteria. This is where the sludge is of outermost importance, since the bacteria are located within the sludge. The sludge is important, since the majority of the cleaning will take place in its presence. The sludge has indeed been a hot topic, not only because of its importance in wastewater treatment, but also of how it is used after.

After serving the useful purpose of filtrating the water and reducing the nitrous compounds in the water, it will be used as fertilisers on crops around Sweden. However, this is where it gets interesting. There are no regulations or laws against any other compounds than nitrous and phosphorous compounds, hence all the other chemical waste in the water ends up in the sludge. Wastewater treatment plants are proud to say that more than 99 % of all microplastics are efficiently reduced in wastewater, but these end up in the sludge instead. The sludge used as fertilisers will therefore have different amounts of pharmaceuticals, microplastics and other pollutants. We thought back on our first brainstorming sessions we held before choosing to work with laccases, and started to think that we had gotten it wrong from the beginning. We thought that the greatest problem is that wastewater has a lot of microplastics and that aquatic animals eat them (which of course is a grand problem), but maybe the biggest problem with microplastics is that it ends up on our crops, and we don’t know the impact of that. But we got reminded by Sofia that our fertilisers have seen to be better than many pesticides and fertilisers that are accepted within the EU.

What we were particularly interested in was where we could implement our enzyme, and we soon got on a long discussion on what step we should focus on. After hearing that the sludge was so important, we thought it as a nice place to install our protein, but since sludge is so thick and it has the possibility to clog filters and other machines, we soon talked about the earlier steps in the plants. A nice idea, we all though, was to target the step with the sand filters, where our beads could be present. When the sand would be cleaned and filtered, our beads would be cleaned with it, therefore avoiding the enzyme to be released into the Baltic sea, which is the final destination of the water being treated at Henriksdal.

It was indeed one of the greatest tours I’ve been to, and she explained each step thoroughly. I could have done without the smell, but seeing all the different steps and how they are combined into a whole made me and all the others excited to go back to the lab and tell all the others about our experience - and the ideas that we had collected during the tour.

Getting back out in the infernal heat did not make us disappointed this time, but it required one gelato to make us fully restored.