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Wastewater
Highlight publication • Water Technology Advisory EU

INSIDE THIS HIGHLIGHT PUBLICATION
Urban wastewater treatment andits potential for the city
Wastewater as a sourceof clean energy
Resource recovery from wastewater

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WASTEWATER
Highlight publication • Water Technology Advisory EU
Version 1.0
December 2023

FRONT PAGE PHOTO
Stiig Larsen
Editing: Henrik Wedel Sivertsen

EDITOR
State of GreenEDITORIAL PARTNER
Water Technology Advisory EU

CONTRIBUTORS
Chapter 1
Author Peter Underlin, Hillerød Forsyning
Case: Varga (DK)
Case: Solrødgaard (DK)
Chapter 2
Author Aarhus Water Per Overgaard Pedersen
Case: NISSEN energy Inc. Thorvald Pedersen
Case: Aarhus Water Per Overgaard Pedersen
Chapter 3
Author VCS Denmark Per Henrik Nielsen
Case: Herning Vand PhosporCare
Case: Billund Vand Thomas Kruse Madsen

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Index

1. Urban wastewater treatment and its potential for the city

4

Case: WWTP contributes to circular economy in the urban water cycle

6

Case: Solrødgård – a fully covered WWTP designed in harmony with nature

7

2. Wastewater as a source of clean energy

8

Case: Achieving 150% energy self-sufficiency

10

Case: Energy positive operations at Downers Grove WWTP

11

3. Resource recovery from wastewater

12

Case: Phosphorus recovery from wastewater

14

Case: Resource recovery for the future

15

3

WASTEWATER

CHAPTER 1

Urban wastewater treatment
and its potential for the city
Globally, 80% of all wastewater is neither collected nor treatedadequately. This poses a significant negative impact on both
environment and human health. Combined with new challenges froma changing climate, urban wastewater management must move fromwastewater treatment to resource and energy recovery.

Urban population growth means wastewater treatment plants(WWTPs) must treat an increasing volume of wastewater in order
to secure the health of the local population and water environment. It is important that both large centralised and small decentralised plants can discharge the treated wastewater withoutharming the recipient, whether it is the sea or a small stream.
Danish WWTPs operate under strict requirements for improved
treatment of wastewater. The wastewater sector has spent many
years refining and developing technologies that treat wastewater
to a high level (through primary, secondary and tertiary treatment)
to ensure that it does not contaminate the recipients. Combinedwith a taxation system which incentivises WWTPs to treat thewastewater beyond the legal requirements, this has resulted inoutlet concentrations which contain far less discharged material(only 20-70 % compared to the legal limits), according to a study
from the Danish Water and Wastewater Association (DANVA).

4

Liveable cities depend on successful wastewater management
Sewage collection and treatment systems play a key role in creating liveable cities where recreational activities are possible along
the city’s harbours, lakes, or rivers. In Denmark, blue-green cities,
harbour baths and other recreational facilities would not havebeen possible without well-managed wastewater infrastructure.
Climate change is putting increasing pressure on wastewater
infrastructure in cities around the world. A common challenge
is to ensure sufficient capacity in the sewer systems to prevent
overflows, especially in times of heavy rain. When redesigning
the wastewater infrastructure, focus should also be on separating and treating stormwater from roads and other contaminatedsurfaces to prevent pollution.  
Powering the city through energy recovery
In alignment with the country’s overall green transition strategy,
the Danish water sector aims to become energy and climate

neutral by 2030. To fulfil this ambition, a green conversion of
the wastewater sector is currently taking place in Denmark. The
starting point has been to focus on energy efficiency and energy
recovery. Today, several plants are now net producers of energy,
where the sludge is used to produce biogas and electricity andexcess heat is used in the district heating system. Recovery ofenergy by using heat pumps is also a new area for innovation anddevelopment.
From wastewater treatment to resource recovery
In the future, wastewater treatment needs to be designed in a
way which allows for not only energy but also other resources(e.g., phosphorous) to be extracted and included in the circular
economy while also ensuring that harmful substances do notreach the recipient. Considering wastewater as a resource is arelatively new perspective and new knowledge and technology is
still needed to recover valuable resources in the wastewater on

a commercial scale. Research and development in Denmark is
dedicated to this field. Advanced membrane technology is alreadyundergoing rapid development, but new specialised technologies
are still needed. Sorption technology that enables collection of
low concentrations of valuable substances or pollutants will also
have a prominent place in the future WWTP. For instance, newbacterial cultures need to be developed to produce base chemicals etc. WWTPs can be seen as production lines, where usable
substances e.g., phosphorus and ammonium are removed along
the way and other products are removed and further processede.g., organic matter to produce biogas or base chemicals which
can be used for high priced products in the pharmaceutical industry etc. Furthermore, wastewater from industries must be carefully reviewed to ensure that valuable components are separated.
Of course, not all substances in the wastewater can be recovered
as some will still need to be removed and degraded for the sake ofthe purity of the products and the discharged wastewater.

5CASES

WWTP contributes to circular economy
in the urban water cycle
Wastewater treatment can contribute significantly to the implementation of circular economy
principles in the urban water cycle of Greater Copenhagen. A new project - VARGA – aims totransform the traditional treatment plant Avedøre WWTP (350,000 PE) into a Water Resource
Recovery Facility, by implementing several innovative technologies within wastewater treatment,
nutrient recovery, and greenhouse gas reduction. Major elements in the project include prefiltration in full scale, P-recovery in pilot scale and active reduction of N2O. As part of the project, a Life
Cycle Assessment and an eco-efficiency analysis has been conducted. Initial results show that
the Total Added Value is higher after implementing the VARGA concept compared to business as
usual – without compromising the treatment efficiency. Circular economy is central for the project
and the experiences gained further contribute to the Danish national goal of an energy and climate neutral water sector in 2030 as well as the UN agenda on sustainable development.

6

CONTRIBUTORSBIOFOS, EnviDan, Unisense Environment,
ARC and DTU Environment with financial
support from the Danish Eco-Innovation
Programme

LOCATION
Copenhagen, DenmarkCASES

Solrødgård – a fully covered WWTP designed
in harmony with nature
The old WWTP in Hillerød was challenged by a growing city and complaints about bad smell
and noise. Building a new traditional plant outside the city would simply cause the problem toreoccur in 30 years. Instead, a 100 % covered WWTP was built in a 52-ha area 4 km from thecity centre. The new ‘Solrødgård Climate and Environment Park‘ is home to all of the utility’s
activities; district heating, waste, water and wastewater. All processes are encapsulated andequipped with advanced DIMS control systems, which provides maximum control of odour andnitrous oxide emissions. Surplus energy goes to the district heating grid and helps displace fossil fuels. The vision was to draw inspiration from nature's integrated systems where everything is
produced from renewable energy sources. The plant’s exit strategy is to leave a well-functioning
wetland in 40-60 years' time, where the biological and recreational aspects are strengthened
compared to today. The master plan is designed with respect for the surrounding nature with thefunctional areas as ‘pockets’ in this contiguous landscape. A new neighbouring district is underdevelopment with a hospital, railway station and 5000 homes and businesses. Real estate prices
are up to 30 % higher than if the plant had been a traditional WWTP.

CONTRIBUTORS
Hillerød Forsyning, Krüger, Stjernholm,
DHI, WSP, Gottlieb Paludan Architectsand Henning Larsen Architects

LOCATION
Hillerød

7

WASTEWATER

CHAPTER 2

Wastewater as a sourceof clean energy
Reducing the costs for collection and treatment of wastewater is
an important issue for water utilities around the world. To achieve
reductions, focus must be on cost efficiency, improvement of thewastewater treatment plants’ energy self-sufficiency and possible
sale of surplus energy to the grid. In Denmark, this means headingfor energy producing wastewater treatment plants and an energy
neutral water cycle.

On a global level, the International Energy Agency (IEA) estimatesthat the water sector accounts for approximately 4% of theworld’s total electricity consumption and wastewater treatment
alone accounts for a quarter of this. Meeting the UN’s target ofhalving the proportion of untreated wastewater by 2030 could
therefore put significant upward pressure on energy demands,
unless energy efficiency and recovery technology is applied at thetreatment facilities. In Denmark, the water sector’s share of thecountry’s total electricity consumption has fallen to 1.9% as more
and more utilities have realised the great potential for energy
savings and energy recovery in wastewater treatment. Most
wastewater treatment plants (WWTP) in Denmark have invested
in an assessment of different ways to reduce their energy consumption. These include implementation of online monitoring andenergy management systems, replacement of surface aeration bymore 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 elec-

8

tricity and heat to the local electricity and heating companies.
Some of the largest water utilities are already well on their way. In
Denmark’s second largest city, Aarhus, the Marselisborg WWTP
produced 30% more electricity than the amount consumed by
the plant itself on average between 2015-2019. At the same time
the treatment plant produced 75% more heat than it consumed,
resulting in a total net energy production of 150%. In Odense
the Ejby Mølle WWTP achieved similar levels of total net energy
production. As a next step, the water utilities in both cities are now
looking into recovering the heat from the wastewater before it isdischarged with the additional benefit of reducing the temperature impact on the receiving waters.
“Danish water utilities will contribute to Denmark’s goal of 70 %CO2 reduction in 2030 through energy recovery and a significant
reduction of energy consumption from wastewater treatment.
At Aarhus Vand, we have reduced our GHG emissions by nearly
80 % since 2008. This shows that there are great benefits to begained by working systematically with energy optimisation.” Lars
Schrøder, CEO, Aarhus Vand and Vice Chairman of the Danish
government’s Climate Partnership on Waste, Water and Circular
Economy.

Solutions depend on plant design and context
More and more WWTPs in Denmark are upgraded with anaerobic
digestion of sludge and/or co-digestion with organic waste products and they utilise the produced biogas to generate electricity
and heat. The optimal solutions depend on the individual plantdesign and the possibilities for either internal use or external saleof the produced electricity or heat. The tipping point for which
the implementation of anaerobic digestion is financially viable
depends on the development of new technologies and changes
in the price structure for purchase and sale of electricity and heat.
In Copenhagen, a technology is currently under implementation
which allows for upgrading biogas to a quality which is similar tonatural gas or vehicle fuel.
Heading for an energy and climate 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 tocover the energy consumption related to its groundwater extraction, water treatment, water- and wastewater transport as wellas wastewater treatment. In 2019, VCS Denmark demonstrated a
100% net energy production for the water cycle in the entire ser-vice area for the utility, covering all 8 WWTPs and the production
and distribution of water in the City of Odense, Denmark’s thirdlargest city with a population of 200,000. The Danish water sector
has set a common goal of becoming energy and climate neutral
by 2030. In 2020, this goal was implemented in the government's
national climate plans.
Benchmarking and innovation lead to lower costs
The innovation of new wastewater treatment optimisation andcost-efficient solutions for both the construction and operationof infrastructure is largely driven by the fact that Danish water
utilities are subjected to mandatory benchmarking on operationalparameters 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. The Danish
Water and Wastewater Association (DANVA) also carries out its
own voluntary benchmarking each year.

9CASES

Achieving 150%
energy self-sufficiency
Over the past five years, the water utility Aarhus Vand has put great focus on energy savings andenergy production. At its Marselisborg WWTP, the utility has implemented energy-saving technologies such as an advanced SCADA control system, a new turbo compressor, sludge liquor
treatment based on the anammox process, as well as optimised the fine bubble aeration system.
This has resulted in a reduction in power consumption of approximately 1gwh/year which corresponds to about 25% in total savings. During the same time period, the energy production has
been improved through implementation of new energy efficient biogas engines (CHP), resulting
in an increase in electricity production of approximately 1 gwh/year. Furthermore, a new heat exchanger has been installed with the aim of selling surplus heat to the district heating grid, which
represents approx. 2 gwh/year. Between 2015 and 2019, Marselisborg WWTP had an average
total energy production of 9.6 mwh/year and an energy consumption of 6.4 mwh/year, equivalent to a net energy production of 150%. Most of the installed technologies have a payback timeof less than 5 years.

10

CONTRIBUTORS
Aarhus Vand

LOCATION
AarhusCASES

Energy positive operations
at Downers Grove WWTP
The Downers Grove Sanitary District in Illinois has dedicated significant resources to reduce its
energy footprint. Improvements in process efficiency including plant automation, aeration system
improvements, upgrades to HVAC and building management systems, and variable frequency
drives have resulted in a 30% reduction in electricity usage at its wastewater treatment plant.
The remaining electricity used by the facility is produced on-site using a biogas driven combined
heat and power system. Biogas is produced by co-digesting hauled food waste and sewage
sludge generated on-site. The biogas is used as fuel to drive an engine-driven electric generator.
Furthermore, heat recovery in the form of circulating hot water is used for plant process heat. The
CHP plant was installed in 2017 with a payback time of 3.5 years. Total Infrastructure investmentsof roughly USD 5 million are expected to have a 10-year payback period. In 2021, the plant produced enough energy to supply its own operations, and feed energy back to the grid.

CONTRIBUTORSNISSEN energy Inc., Landia and Downers
Grove Sanitary District Wastewater
Treatment Center

LOCATION
Illinois, USA

11

WASTEWATER

CHAPTER 3

Resource recovery fromwastewater
Traditionally, wastewater has been considered a liability, meeting
increasingly stricter standards for wastewater discharge withincreases to the costs of treatment. Utilising the resources in thewastewater can prove an important step in the opposite direction,
considering wastewater treatment plants as resource recovery
facilities.

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 suchhigh standards that it can be reused in a number of ways – e.g. forflushing toilets or laundry machines.
Utilising organic content
As described in the previous chapter, organic material in wastewater can be separated and utilised for biogas. This has beenstandard procedure in larger wastewater treatment plants for awhile, and new water treatment technologies and more efficient equipment for combined power and heat production haveincreased the potential. Organic content can be saved for energy
use in biogas production if new carbon saving processes fornutrient removal are introduced. Denmark has vast experience in
optimising the use of carbon and is now also gaining know-how innutrient recovery.
Phosphorus recovery from wastewater sludge
Phosphorus is a scarce resource with great value for the agricultural sector. Phosphorus is accumulated in the wastewatersludge and in internal side streams and if treated properly, it is12

possible to change this into a controlled harvesting of a purefertiliser. 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 withheavy metals and micro pollutants from wastewater in the sludge.
The phosphorus product struvite has been approved in Denmark
as a fertiliser product. Two full-scale plants in Aarhus currently
forms the background to increase the current P-recycling fromapproximately 15% to 25%. Once these are completed, the total
phosphorus recovery throughout the catchment area is expected
to be increased to approximately 22 tonnes P/year or approx. 0.5
tonnes of struvite fertiliser per day.
 enefits of using struvite fertiliser compared to sewage sludge
B
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. The content
of the typical problematic metals such as lead, cadmium,nickel, chromium copper and zinc is a factor of 20-100 times
lower in relation to the content of phosphorus.

•	Reduced risks of groundwater contamination: Phosphorus
from wastewater can be utilised for agriculture without
risking a contamination of soil and groundwater with theaccumulation of heavy metals and other harmful substances to the environment, which makes it possible to carry out
subsequent groundwater exploitation in the same area.
•	Greater flexibility in terms of usage and storage: Struvite ismuch 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)•	Ready-to-use as fertiliser: There is no need for further
processing as the material is ready to use and can be mixed
with other mineral fertilisers if there is a need for changing
the level of potassium or nitrogen.

•	Low solubility: Struvite has a low solubility, making it suitable for depot fertiliser where the phosphorus content isreleased slowly in line with the needs of the plants. This is anadvantage for fertiliser spreading without danger of dissolution into groundwater or surface water.
•	Lower cost: Sludge from P-recovering plants, which is low
on phosphorus, can be used as a bio-fuel without it resulting
in a loss of phosphorus to ashes. The cost of regaining phosphorus from ash is much higher than extraction as struvite
from wastewater.
Struvite based P-recovery is the state-of-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.

•	Better suited as dedicated fertiliser: Struvite has shown
excellent properties for fertilising specific plants and crops
with need for extra phosphorus and magnesium. Golf courses and plant nurseries are good examples of this.

13

CASES

Phosphorus recovery from wastewater
In 2015, the water utility Herning Vand Ltd. Opened the second P-recovery plant in Denmark,which recovers phosphorus from a concentrated side stream in the wastewater treatment plant.
For several years, the WWTP suffered from struvite scale build-up in its sludge and wastewater pipes, causing problems for the dewatering process of sludge and biogas production. In
addition to solving the problem, Herning Water wanted to exploit the potential of recycling the
struvite into agricultural fertiliser. A solution based on controlled precipitation of struvite wastherefore designed, and a full-scale recovery plant of the phosphorus compound struvite wasbuilt based on previous test results from Aarhus Vand at its plant Aaby WWTP. At both plants, thestruvite is precipitated as a ‘ready-to-use fertiliser’ and sold to a fertiliser company. An official
approval of the product as commercial fertiliser has been obtained for the struvite produced atboth the Herning and Aarhus plants under the name Phosphorcare™. The operational savingsat the treatment plants and the expected revenue from sale of struvite is expected to result in apayback time of 10-12 years.

14

CONTRIBUTORS
Herning Vand, Aarhus Vand, Stjernholm,
Grundfos, Norconsult, Suez and SEGESLOCATION
Herning and Aarhus, DenmarkCASES

Resource recovery for the future
Billund Biorefinery (BBR) is a resource recovery plant that integrates waste management andwastewater treatment. BBR produces clean water, energy for the local public district heating andpower grids, as well as high quality natural fertiliser for the surrounding agricultural areas. The
wastewater catchment areas consist of combined and separate sewer systems and the wastecomprises of sorted organic waste from households and local industries. BBR integrates wastewater treatment with anaerobic digestion and other innovative processes like Exelys™ (thermalhydrolysis) and Anitamox™ (Anammox process). These, along with STAR™ advanced online
monitoring and control system, minimises energy usage and maximises energy production andeffluent quality. As a result, effluent nutrient concentrations (N, P and COD) have been reduced toa quarter of the level required by Danish legislation and the plant operates with a 200% energy
surplus relative to the plant’s own consumption. BBR is a public private partnership and was
financially supported by the Danish Eco-Innovation Programme (MUDP) and the Danish Water
Sector Foundation (VTUF).

CONTRIBUTORS
Billund Vand og Energi A/S and Krüger
Veolia A/S

LOCATION
Billund, Denmark

15

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