Reducing the costs for collection and treatment of wastewater is an important issue for water utilities around the world. In order to achieve this, focus must be on cost efficiency, improvement of the wastewater treatment plants’ energy self-sufficiency and possible sale of surplus energy to the grid.

Most wastewater treatment plants in Denmark have invested in an assessment of different ways to reduce their energy consumption. These include implementation of online monitoring and energy management systems, replacement of surface aeration by more energy efficient bottom aerators and different operational approaches.

New focus towards energy self-sufficiency
In the recent years, Danish water utilities have moved beyond simply focusing on reducing energy consumption, to also focusing on energy production. The first goal is typically to become energy neutral, and the second goal is being able to sell excess electricity and heat to the local electricity and heating companies.

Some of the largest water utilities are already well on their way. In Odense, Denmark’s third largest city, the Ejby Mølle WWTP from VCS Denmark achieved 180% energy self-sufficiency (see how in the video). As a next step, the water utilities are now looking into recovering the heat from the wastewater before it is discharged with the additional benefit of reducing the temperature impact on the receiving waters.

Solutions depend on plant design and context
More and more wastewater treatment plants in Denmark are upgraded with anaerobic digestion of sludge and/or codigestion with organic waste products and they utilise the produced biogas to generate electricity and heat. The optimal solutions depend on the individual plant design and the possibilities for either internal use or external sale of the produced electricity or heat. In Copenhagen, a new technology is currently under implementation which allows for upgrading biogas to a quality which is similar to natural gas or vehicle fuel.

Heading for an energy neutral water cycle
By introducing new technologies to reduce energy consumption and improve energy production, it is the goal that the utility companies can provide an energy neutral water cycle. In this scenario the energy production from the utility’s treatment
plants is able to cover the energy consumption related to its groundwater extraction, water treatment, water- and wastewater transport as well as wastewater treatment. In 2015, Aarhus Water demonstrated a 90% net energy production for the
water cycle in Marselisborg catchment area (Aarhus city centre).

Benchmarking and innovation lead to lower costs
The innovation of new wastewater treatment optimisation and cost-efficient solutions for both the construction and operation of infrastructure is largely driving by the fact that Danish water utilities are subjected to mandatory benchmarking on operational parameters and cost efficiency across the water sector. Innovation projects are often based on collaboration across governmental bodies, water utilities, consulting companies, technology suppliers, universities and research institutions.

Source: “Unlocking the Potential of Wastewater” by State of Green

Traditionally, wastewater has been considered a liability. Meeting increasingly stricter standards for wastewater discharge also increases the costs of treatment. Utilising resources in the wastewater can prove an important step in the opposite direction.

Considering wastewater as a resource is a relatively new perspective, however today it is widely recognised that the organic content in the wastewater can be a resource for energy production, the nutrients – especially the phosphate – can be used for fertiliser production, and the water itself can be cleaned to such high standards that it can be reused in a number of ways – e.g. for flushing toilets or laundry machines.

Utilising organic content

Organic material in wastewater can be separated and utilised for biogas. This has been standard procedure in larger wastewater treatment plants for a while, and new water treatment technologies and more efficient equipment for combined power and heat production have increased the potential. Organic content can be saved for energy use in biogas production if new carbon saving processes for nutrient removal are introduced. Denmark has vast experience in optimising the use of carbon, and is now also gaining know-how in nutrient recovery.

Phosphorus recovery from wastewater sludge
Phosphorus is a scarce resource with great value for the agricultural sector. Phosphorus is accumulated in the wastewater sludge and in internal side streams and if treated properly, it is possible to change this into a controlled harvesting of a pure fertiliser. The recovery of phosphate for fertiliser enables a multitude of possibilities for sludge handling, not wasting the valuable phosphorus to end up in low quality form as ashes or mixed with heavy metals and micro pollutants from wastewater. The phosphorus product struvite has been approved in Denmark as a fertiliser product.

Benefits of using struvite fertiliser compared to sewage sludge
The solution of recovery of struvite as a pure mineral phosphorus fertiliser offers several advantages in comparison to the application of sewage sludge on agricultural land:

• Environmental benefits: Struvite is significantly cleaner than the sewage sludge in terms of heavy metals.
• Reduced risks of groundwater contamination.
• Greater flexibility in terms of usage and storage: Struvite is much more flexible as a fertiliser as the material is concentrated, comes in a dry form and is possible to store for longer periods of time.
• Economic benefits: Struvite can be sold at a high price (up to EUR 335 per tonne)
• Better suited for fertiliser use: Sludge from wastewater treatment plants, which has recovered phosphorus, have a lower content of phosphorus relative to nitrogen.
• Low solubility: This is an advantage for fertiliser spreading without danger of dissolution into groundwater or surface water.
• Lower cost:  The cost of regaining phosphorus from ash is 5 to 10 times higher than extraction as struvite from wastewater.

Struvite based P-recovery is the stateof-the-art for phosphorus recovery from wastewater. The technology is still under development and Danish wastewater utilities and companies are working on developing even more efficient process solutions. 

Source: “Unlocking the Potential of Wastewater” by State of Green

In many countries, decades of uncontrolled wastewater discharge have resulted in severe environmental degradation of both inland and coastal waters. Stricter regulatory standards for both treatment efficiency and effluent quality are important tools for reversing this trend.

Regulation as a driver for development
The environment has been a key focus area in Denmark for a long period and some of the oldest wastewater treatment facilities were established more than 100 years ago and today, 95% of all wastewater is treated.

Denmark was among the first to take major steps in minimising the adverse impact from the cities’ wastewater discharge to the aquatic environment. The approach has been to use novel technologies and not just move the pollution away from the cities but treat the sewage water from the cities to an appropriate standard. This has enhanced the liveability in the Danish cities and made the surrounding areas more attractive.

Geographically, Denmark is challenged by the lack of major water bodies for discharge of pollutants from the cities. As a result, the environmental aspect has been important for a long time and has led to stringent regulation implemented since the 1980s, with the Danish standards for discharge to sensitive waters being further strengthened during the 1990s.

Much of the legislation passed in Denmark has been taken up almost in its full content by the European Union and today, Denmark is subject to the EU Water Framework Directive.

However, the Danish regulation is in many aspects more stringent than the general EU requirements. From the beginning, the legislative requirements and standards have been based on a specific evaluation of the recipient with the aim of ensuring vast improvements of the water quality in the surrounding water bodies.

The result has been a substantially improved aquatic environment with many cases of water quality enabling salmon breading in rivers and creeks – even in very densely populated areas. The recreational value of the Danish water bodies has also been improved and in many cities it is now possible to swim in the inner city harbours.

Source: “Unlocking the Potential of Wastewater” by State of Green

Municipal wastewater treatment plants are mainly designed to remove easily degradable organic substances and nutrients which are the major constituents of domestic wastewater. However, in many countries the sewer network and treatment plants often also receive wastewater from industrial production.

Industrial wastewater differs a lot
Wastewater from food processing companies is generally well suited for centralised treatment since it is often rich in easily degradable organic compounds, which provide nutrients for the growth of microorganisms and thereby enhance the biological processes. In fact, some treatment plants actively encourage discharge of waste from food processing industries because it enables them to increase the biogas production and thereby generate more energy, as well as improves the biological removal of nitrogen and phosphorous.

Wastewater from manufacturing industry, on the other hand, has a much more complex composition and often includes substances that do not respond to biological treatment or which may hamper the growth of microorganisms and therefore interferes with the biological treatment processes or makes the sludge unsuitable for use in agricultural fertilisers.

More efficient to treat at the source
Treatment of industrial wastewater at the source has several advantages. First and foremost, treatment can be tailored to the specific industrial pollutants which typically occur in high concentrations in relatively small volumes of wastewater and therefore requires relatively low investment and operational costs.

More importantly, it may be possible to recycle or reuse wastewater internally in the production after partial treatment or to recover and reuse raw materials or chemicals.

Methods for decentralised treatment
Industrial pollutants, which require specialised treatment processes, include non-degradable or slowly degradable organic compounds, fat, grease, oil, heavy metals and toxic organic compounds such as pesticides and pharmaceuticals.

A number of specialised processes for removal of industrial pollutants can be applied at the source, including neutralisation, heavy metal precipitation, membrane filtration, activated carbon filtration, chemical oxidation, ultra-violet disinfection, evaporation, crystallisation and more.

Source: “Unlocking the Potential of Wastewater” by State of Green