Why should we care about wastewater?

In 2015, 4.9 billion people used an improved sanitation facility while 2.4 billion did not. Click here to learn more about the water and sanitation situation worldwide.

The implementation of safe and efficient wastewater treatment systems is essential to satisfy global water demand, safeguard the environment, protect public health and meet sustainability goals. Click here to learn more on the 17 sustainability goals of the United Nations.

Energy-positive wastewater treatment plants (WWTPs) create an enabling environment for greener, smarter and more circular cities. Click here to read the UN World Water Development report 2016.

The Power Plant

Municipal wastewater treatment in Europe requires a lot of energy to eliminate organic matter and nutrients from sewage, prior to its discharge.

The electricity demand for a conventional WWTP operation represents a major cost driver for WWTP operators and can make up to 20% of a European municipality’s electricity bills.

Around 1% of the EU’s electricity demand is consumed by conventional WWTPs per year, which is equivalent to 2 large power stations, or 4 million private households.

The City

With high levels of the EU population living in urban areas, sewers connected to public wastewater treatment plants collect a significant fraction of wastewater.

Treatment of municipal wastewater has improved in all parts of Europe. Today, more than 90% fulfil the necessary requirements for discharging into receiving bodies of water.

Despite improvements, WWTPs are the main energy consumers in EU cities.

Raw wastewater

Wastewater from households and industry contains a lot of organic matter, nutrients and hazardous substances that put significant pressure on hydrological environments.

However, organic matter in wastewater could be utilised due to its significant energy potential. For the 500 Millions EU citizens, it would represent 87,500 GWh per year, which is equivalent to the annual output of 12 large power stations or the electricity demand of 4 Millions private households.

Click on « upgrade your city » to discover the wastewater treatment plant of the future!

Primary treatment

Primary treatment of raw wastewater consists of a physical sedimentation process to separate larger particles (i.e. sludge). The remaining liquid is either discharged or put through a secondary treatment process.

Here, around 30% of organic pollution can be separated using simple sedimentation tanks.

Generated sludge is a mixture of water (around 95%) and larger particles or organic and inorganic matter (e.g. sand).

Activated sludge process

After primary treatment, wastewater is treated in a biological process called “activated sludge process”.

Here, microorganisms living in the ‘activated sludge’ eat up the organic substances and nutrients.

By eating up these pollutants, microorganisms grow and build up biomass within the ‘activated sludge’.

Air and C02

The microorganisms living in the ‘activated sludge’ need lots of oxygen to breathe and the respective air-blowers of a WWTP plant consume around 2/3 of the energy demand.

Organic pollution of the wastewater is either converted to CO2, just as humans produce, or into biomass.

Clarifier

Finally, biomass is separated from purified water in large sedimentation tanks.

While a part of this biomass is recycled back to the activated sludge tank, excess biomass is sent to sludge treatment (“excess sludge”). Purified water can be discharged safely to surface waters or oceans.

Effluent

Purified wastewater is water with a very low content of residual organic matter and nutrients. It can be discharged or reused for other purposes (e.g. agriculture, landscape irrigation) if suitable post-treatment is applied.

The EU has set some rules on wastewater quality. The objective of the Urban Waste Water Treatment (UWWT) Directive is to protect surface waters from the adverse effects of wastewater discharges.

Anaerobic digestion

Sludge from wastewater treatment has to be stabilized to prevent it from uncontrolled degradation which emits bad odours and other harmful gases. Stabilisation can be realized in anaerobic digestors.

This process works not with oxygen but with heat (38-55°C) and degrades half of the organic matter in the sludge into biogas (65% methane and 35% CO2).

Combined Heat and Power plant

Combined Heat and Power (CHP) plants convert biogas into electricity and heat.

CHP plants convert 30-40% of biogas into electricity and 40-50% into usable heat, while 20% are lost as waste heat.

Based on the theoretical energy potential in raw wastewater, a conventional WWTP with CHP recovers around 10% of this energy potential as electricity (including losses in the overall process steps).

Dewatering

After anaerobic digestion, the residual sludge is separated into liquid and solids. This process is called “dewatering”.

Solids are usually disposed of in agricultural processes or through incineration.

Liquids are recycled back to the influent of the WWTP.

POWERSTEP

The EU project POWERSTEP aims to convert wastewater treatment plants (WWTPs) into power production facilities. It demonstrates how they can become viable energy producers without compromising the water treatment quality or cost.

Energy-positive WWTPs help cities become smarter, greener, save money and truly become a circular economy!

Having started in July 2015, POWERSTEP is based on 6 full-scale case studies that bring together 15 partners from 7 different European countries.

POWERSTEP

EU municipal wastewater can be a real resource for cities. The organic matter it contains has a chemical energy potential of 87,500 GWh per year, which is equivalent to the electricity produced by 12 large power plants, or the electricity consumed by 22 million private households.

As WWTPs are well connected to energy supply networks, upgrading WWTPs will not require additional power infrastructure. It might be the closest renewable energy source a smart city can exploit!

The power plant

Thanks to POWERSTEP technologies, the electricity demand of wastewater treatment will be significantly reduced. Overall, WWTPs can even become energy positive in their total energy balance.

With POWERSTEP, WWTPs can become a source of renewable energy for cities and act as a renewable power plant!

What is it about?

At the WWTP of Westewitz (Germany), POWERSTEP implements a new technology that helps to separate more sludge from the raw wastewater. This sludge contains valuable organics (i.e. carbon), which is fed into the anaerobic digester to produce biogas.

How do we do it?

The process for carbon extraction is based on a new filtration technology (micro-screen). The principle is to use fine mesh to separate more particles from the liquid. To enhance the extraction efficiency, chemicals can also be added before the filtering process.

What are the expected results?

This strategy should lead to carbon extraction rates of up to 70-80% as well as full automatic operation and cleaning of the micro-screen.

What is it about?

After enhanced carbon extraction in A-stage, biological removal of nutrients is a challenge. Therefore, a specific strategy is required to operate biological nutrient removal in B-stage and fulfil the water quality criteria at the end of the treatment.

How do we do it?

At Westewitz, different strategies are tested based on online sensors and real-time control as well as new technologies like compact duck-weed reactors.

What are the expected results?

The goal is to reach stable removal of nutrients after enhanced carbon extraction to fulfil legal nutrient discharge limits.

This case study will also show that the overall concept of A-stage and B-stage that use enhanced carbon extraction can be operated fully automatically.

What is it about?

As for the WWTP of Westewitz (case study n°1), POWERSTEP implements a new technology that helps to separate more sludge from the raw wastewater at the WWTP Sjölunda (Sweden). Enhanced carbon extraction will be tested with a micro-screen as a pre-filter.

How do we do it?

At WWTP Sjölunda, a different micro-screen configuration to the one used in Westewitz is operated. This will enable a comparison of different micro-screen configurations and also a replication and validation of the technology at two different sites.

What are the expected results?

A comparison of micro-screen technology versus existing primary treatment at WWTP Sjölunda.

A comparison of different micro-screen configurations in performance, efficiency of carbon removal, and operational stability.

What is it about?

As for case-study n°1, biological removal of nutrients is a challenge after enhanced carbon extraction at A-stage. Therefore, a specific strategy is required to operate biological nutrient removal in B-stage and fulfil the water quality criteria at the end of the treatment.

How do we do it?

At the WWTP of Sjölunda, a new biological process to remove nitrogen is tested called “Mainstream deammonification”. It consists of two specific steps of biological treatment, which need less energy for aeration and no carbon.

What are the expected results?

The objective is to assess whether this new biological technology can be operated stably during main-stream treatment, and at low temperature (i.e. winter seasons).

What is it about?

At Avedoslash;re WWTP (Denmark), a biological methanation plant has been built that upgrades biogas (60% CH4) to natural gas (> 95% CH4). This high quality gas can then be injected into the National Gas Grid as renewable energy.

How do we do it?

A biological methanation stage is used where special bacteria convert carbon dioxide (CO2) and hydrogen (H2) into methane (CH4). The hydrogen is supplied by an electrolyser which splits water (H2O) into hydrogen (H2) and oxygen (O2) by using electricity. This technology concept is therefore called “Power-to-Gas”.

What are the expected results?

Converting “low-grade” biogas into “high-grade” natural gas that can be stored as renewable energy in the gas-grid.

Showing cost-effective operation of Power-to-Gas technology in combination with a WWTP that integrates the usage of by-products (e.g. heat and oxygen) on-site.

What is it about?

POWERSTEP aims to increase the energy autonomy of the Braunschweig WWTP (Germany) through innovative energy and heat management strategies. This implies Heat-to-Power technologies for improved heat usage and smart grid strategies for integration of WWTP into the electricity grid.

How do we do it?

High-temperature excess heat from the Combined Heat and Power plant (CHP) will be converted into electricity using thermoeletrical modules. Thermoelectric materials generate power directly from heat by converting temperature differences into electric voltage.

Dynamic mathematical models of electricity consumption and production profiles are created to optimize WWTP operation and find the best option for selling or buying electricity from the grid.

What are the expected results?

The main objectives are the demonstration of Thermoelectical modules in large-scale and the testing different smart grid strategies for practical implementation from an economic point of view.

What is it about?

Sludge water after anaerobic digestion is highly polluted and has to be recycled via the entry of the WWTP. This side-stream can be treated separately before recycling to minimize the overall energy consumption of the WWTP.

How do we do it?

At WWTP Kirchbichl (Austria), an alternative biological side-stream treatment process is tested to optimize the overall process as well as the electricity consumption.

Existing tanks for side-stream treatment will be operated with a different strategy to establish the alternative treatment.

What are the expected results?

A stable side-stream treatment with low energy consumption should be realized. The reduction of greenhouse gas emissions in main-stream as well as side-stream treatment should also be reached. Overall, the energy balance of the WWTP should be optimized.

What is it about?

Sludge water after digestion is highly polluted and has to be recycled via the entry of the WWTP. This side-stream can be treated separately before recycling to minimize the overall energy consumption of the WWTP.

At WWTP Altenrhein (Switzerland), an alternative physical side-stream treatment process is tested to optimize the overall process as well as recover nutrients for fertilizer application.

How do we do it?

The side-stream treatment is based on using an innovative membrane technology to separate nutrients from sludge water. Nitrogen in the form of ammonia gas can diffuse through the gas-permeable membrane and is collected in an acid solution, which can then be used as fertilizer.

What are the expected results?

Treatment of ammonia in sludge dewatering effluent will save on aeration demand and will offer the opportunity to recover nitrogen in the form of a fertilizer product.

Polishing

If the legal limitations that protect surface waters from WWTP discharge cannot be safely reached after B-stage, a polishing step is required to guarantee that the EU legal standards for organic matter and nutrients in WWTP effluent are met.

Effluent

The main objective of the Urban Waste Water Treatment Directive is to protect surface waters from the adverse effects of wastewater discharges.

The energy optimisation of future WWTPs should never compromise the performance in terms of effluent quality, which remains the primary target for all WWTP processes.

Anaerobic digestion

Sludge from wastewater treatment has to be stabilized to prevent it from uncontrolled degradation, emitting bad odours and other harmful gases. Stabilisation can be realized in anaerobic digestors.

This process works not with oxygen but with heat (38-55°C) and degrades half of the organic matter in the sludge into biogas (65% methane and 35% CO2).

With POWERSTEP, the biogas yield of anaerobic digestion will be increased by 80-150% for the overall WWTP. It can then be valorised through different routes!

Natural Gas

Make the city buses run thanks to wastewater!

Thanks to an Energy-positive WWTPs upgraded biogas (>95% CH4) will be directly injected into the local/national gas grids or used to fuel vehicles.

Electricity

Lighting up a city!

A city energy-positive WWTP will produce and sell electricity instead of paying for it. Wastewater will be a renewable energy source for the (smart/green/sustainable) city!

Heat

Blowing heat!

Energy-positive WWTPs can provide heat (e.g. district heating). It might just be the closest renewable source a smart city can exploit!