How can we help improve our wastewater?

165 litres of water are consumed on average per day per person. After being used for laundry, flushing the toilet, cleaning, cooking, etc., , wastewater is evacuated through the sewage networks. Before wastewater is returned to nature, polluting elements must be removed, whether organic or chemical. Patrick LICKER, head of the Service Canalisation (Sewer Department) for the City of Luxembourg, explains the importance and operation of a wastewater treatment plant.

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1. Why is it important to transport wastewater produced by households and industry to treatment plants?

Wastewater collection and transport to treatment plants are essential to ensure public cleanliness and hygiene while ensuring the comfort of residents and users in urban areas.

We all tend to take it for granted that these processes run smoothly, without much concern. It is only when a malfunction occurs, such as a clog or bad odours, that we realise the value of a well-functioning sewage system.

Effective wastewater treatment in a high-performance treatment plant is of vital importance in terms of environmental protection. At the City of Luxembourg’s treatment plant, more than 1,000 tonnes of waste and 7,000 tonnes of sewage sludge are separated from wastewater each year and, at the same time, biological treatment at the plant eliminates most of the organic pollution dissolved in wastewater. The proper functioning of municipal wastewater treatment stations is an essential link for the development and preservation of the flora and fauna of our waterways.

Moreover, given the demographic expansion in our country, the performance requirements of treatment plants are growing, resulting in increasingly restrictive water discharge thresholds imposed by the competent state authorities.

2. What is the strategy for rainwater management?

The purpose of a municipal treatment plant is not to treat rainwater. In the case of a split network, rainwater is not mixed with wastewater but is directed to a separate network and thus encouraged to flow into the waterways. Rainwater treatment is only planned in certain cases, such as the treatment of run-off water from major high-traffic roads.

More than 60% of the City of Luxembourg’s sewage network operates in a split system.

For the combined sewage system, storm drainage systems must be put in place at the various spillways in order to separate a certain amount of pollution from mixed water before it flows into the waterways.

The implementation of new urban development projects automatically requires the use of the split collection system, as well as the systematic application of technical solutions for infiltration and retention of rainwater.

3. Could you briefly explain the operation of a treatment plant?

The operation of a municipal wastewater treatment plant consists of 3 steps:

mechanical pre-treatment,
primary treatment and
biological treatment.

Mechanical pre-treatment consists of screening, degritting and degreasing. The last 2 steps take place, in general, in the same set of basins. Pre-treatment therefore eliminates solid waste, sand and grease.

The primary treatment that follows uses large settling tanks to isolate and evacuate sewage sludge.

Finally, biological treatment eliminates organic pollution in solution in the wastewater, whether it is carbon-based elements or nutritive elements. These are comprised of nitrogen and phosphorus.

This step takes place in biological reactors, where dissolved pollution is metabolised by various micro-organisms.

4. What happens with the sewage sludge and what is the connection rate to treatment stations?

Sewage sludge is mostly produced during primary settling, with a smaller portion coming from the biological reactor. This mixture first undergoes a fermentation stage for 28 days at a temperature of 38°C in the digestion towers. At the Beggen treatment plant, each year, sludge digestion produces some 1,8 million m3 of recovered biogas energy in a cogeneration plant. The digested sludge is then mechanically dehydrated before being evacuated. In Beggen, two large sludge containers are evacuated every day, or around 7.000 tonnes per year.

In recent years, the agricultural recovery of sludge as fertilizer is unfortunately no longer feasible on a large scale.

Today the sludge is incinerated in industrial furnaces or charcoal kilns, cement kilns or household waste incinerators. Almost all sludge is exported to neighbouring countries.

The lack of evacuation solutions in the Grand Duchy of Luxembourg is a major concern for treatment plant operators. Currently, studies are underway, under the supervision of the Ministry of the Environment, Climate and Sustainable Development to establish national solutions, considering the installation of sludge incineration plants, which produce phosphorus-rich ash. Technical processes are currently being developed to recover the phosphorus from the ash in the future. Of the around 19.000 buildings in the City of Luxembourg, only about fifteen are not connected to the public sewage network, as connection is technically not feasible. These buildings have waterproof tanks. The network connection rate is therefore 99,9%.

5. What are your five tips on how to contribute to the proper functioning of wastewater collection and treatment?

The sewage pipes are not a waste bin! Some products may interfere with the operation of the treatment plant, damage or clog the pipes or chemically harm the pipes and infrastructure.

5 useful tips:

1. Avoid throwing or pouring into the pipes: kitchen waste, chemicals, oils, fats, medicines, sanitary napkins, wipes, tampons, condoms, etc. These products should be disposed of as waste.

2. Do not throw anything into storm drains as these products may end up in waterways.

3. Do not throw cigarette butts on public roads for the same reason.

4. Don’t wash your car on public roads or your private driveway as these surfaces are probably directly connected to the waterways.

5. Contact the “Bureau de raccordements” (connection office) if you have a construction or real estate transformation project in the territory of the City of Luxembourg. We will advise you when drawing up plans for sanitary facilities and will ensure that everything is connected correctly.

About the blog:

There is an urgent need for rapid transition to global sustainability. Business and industry have enormous social and environmental impacts. "Why does it matter?" is a bi-monthly blog that aims to elucidate this important topic through the eyes of our experts.

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