Part 3. Sequester — redefining wastewater

One of the most encouraging aspects of the Danish water supply is found in its having fostered some of most compelling displays of sustainable wastewater management in the world. At several wastewater treatment plants (WWTPs) around the country solutions are already in operation securing results that just a decade ago might have been dismissed as outlandish.

To be sure, of the hundreds of foreign water industry stakeholders who visit these facilities every year, most leave in clear awe of what they see and learn.

There is good reason for the favorable impression that people depart with. Not only have many facilities demonstrated routes to securing energy neutrality — a claim many more Danish plants are working towards — a number are significantly net positive. This happens from a combination of energy consumption reduction initiatives and onsite energy production capabilities. 

Remarkably, Marselisborg WWTP in Aarhus produces up to 70% more energy than it uses. Energy is sent to consumers in the form of green electricity and heat; making Marselisborg the world's most energy efficient treatment plant.

Here, the generation of power from anaerobic digestion of sludge is complemented with production of heat which is supplied to district heating networks. It is a remarkable state of affairs given that within just a lifetime, wastewater treatment within many of those same municipalities represented the largest single source of energy consumption in their region.

And yet the story does not end there. While nascent in its development, considerable gains have been made in the field of resource recovery from wastewater streams. 

Presently, various systems are in place at facilities trialing and refining techniques for sequestering nutrients whilst also producing other soil enriching by-products for the agricultural industry.

Jens Munk-Poulsen, manager of wastewater in Skanderborg Utility and AquaGlobe – Water Solution Center, provides outlook on the themes driving innovation across the Danish wastewater scene: “The circular economy is making a huge entry into wastewater treatment, alongside environmental protection and energy production. Altogether, energy efficient smart water technology, coupled with energy production technology changes plants in a fundamental manner. It’s such an obvious business case bringing down CO2 emissions, it hurts to see it not being adopted the world over.” 

From net-neutral to net-positive

Key to WWTP self-sufficiency has been reducing energy consumption in the first instance. Recalling that part two of this series explored improving energy efficiency across supply networks, great advances have also been made at treatment facilities to reduce process energy demands.

Aarhus Vand is water utility to Denmark’s second largest city, providing water supply, storm- and wastewater services to Aarhus municipality and its 350,000 residents. The utility operates four large WWTPs that collectively receive approximately 35 million cubic meters of wastewater a year. 

In recent years, Aarhus Vand has engaged in a radical overhaul of its assets geared towards delivering a more sustainable ecosystem of infrastructure and processes. Against a backdrop of seemingly contradicting objectives — including, expanding capacity, reducing effluent volumes and energy consumption, whilst also delivering on resource recovery targets — it is remarkable to report on Aarhus Vand's successes.

Pia Jacobsen, chief engineer – water reuse Aarhus Vand, explains: “Our resource strategy is very ambitious. For instance, a target is to reach energy production at Egå WWTP similar to what we’ve achieved at Marselisborg. When we talk about CO2, by 2030 we aim to be 100% energy and CO2 neutral across the whole water cycle. There are also goals for increasing phosphorous recovery — the goal is to ramp up to produce 200 tons per year across Åby and Marselisborg.”

A critical part of the solution to these net-positive ambitions was found in process optimization, involving real-time monitoring of processes, and systems that automatically fine-tune processes in response to variable conditions.

Process optimization at Aarhus Vand was very much a result of collaboration with Danish water solutions expert, DHI. Jacobsen comments: “Traditionally processes were built very safely, without concern for energy consumption. Now we’re working much more closely to the processes and controlling them more finely. Our plants are also mostly fully automated.”

Jacobsen continues: “Enabled by new technologies which provide insight on what’s actually happening, we can, for instance, optimize aeration for specific processes and not just to keep to a certain oxygen level in the water. Another example is optimizing basin capacities, by equalizing across basins to avoid bottle-necks.”

Jacobsen highlights a DHI Solution Software called Data Integration and Management System (DIMS), which functions on top of the existing supervisory control and data acquisition/programmable logic controller (SCADA/PLC) systems. Moreover, it utilizes new sensors (including ones monitoring ammonium-, nitrate-, and phosphate) to enable calculated set point control and automated optimization of processes, whilst avoiding major construction extensions.

These deployments in mind, Jacobsen cautions that we should not underestimate the importance of high-quality hardware: “The opportunities of advanced SCADA require particular physical solutions, for instance variable valves, gate controls and so on. For this it’s been critical to work with companies such as AVK to develop new solutions.”

Clear outcomes

With process optimization systems in place at Aarhus Vand’s four major plants, treatment capacities increased, while energy and chemical consumptions lowered. The plants lowered their CO2 emissions and ultimately their collective environmental footprint. Effluent values decreased too and are now highly predictable according to given ammonium and phosphate set-points.
In terms of energy output through 2018, Marselisborg produced 4,552 MWh electricity and 4,676 MWh heat.

Collaborative atmosphere

What is clear from the Danish perspective is the value of partnership and cooperation. Jacobsen remarks: “In Denmark, collaboration between public and private actors has been in play with some years. It’s been key to our success and I only see it as increasing. We need to collaborate — no individual company can deliver the solutions the global industry requires.'

DHI’s Ottosen agrees: “Our approach is very much to deal and plan for the unknowns and become more robust and resilient; and to find new synergies — that requires perspective from multiple stakeholders.”

He continues: “Collaboration isn’t only leading to novel solutions, it reduces costs. Delegations that visit Hillerød (a plant fully integrated within a public park) often remark that they simply don’t understand its cost of 265 million DKK ($40 million). If that were built in the US they expect it would cost about $265 million! It makes it clear for me, how important it is that we share these solutions and knowledge.”

Centralization of WWTPs 

Against this background of novel technologies being implemented, a notable trend is centralization of plants. 
For instance, Skanderborg Utility is looking to moving from six facilities to two or three WWTP in the future.

Poulsen comments on aspects of the rationale for this saying: “Many utilities are working towards energy neutrality, but towards energy production, many plants lack the critical mass for digesters to be effective or profitable. Centralizing WWTPs, to fewer but larger plants incorporating supply plus treatment, will improve the situation.”

In considering the options when replacing aging infrastructure, a concern of prospective customers is an expectation of high upfront capital costs. But there is a silver lining, as Poulsen explained: “Energy efficiency measures are easier to get going first and can deliver significant results; we saved 10% in operational expenditure through smart water technology and 5% on wastewater treatment between 2017 and 2018. In 2018, in another WWTP we saved 28% through energy efficiency, with 75% savings on aeration alone.”

Centralization is also underway in Aarhus, and a new, larger plant is in the planning. Jacobsen described the forthcoming Aarhus ReWater as a developing plant, “open to new technologies from day one with a modular construction, so that new technologies can be implemented.” 

Aarhus ReWater is pitched to become one of the most advanced treatment and resource plants in the world when it comes online 2026.

A first-mover on resource recovery

A final notable aspect to the Danish wastewater scene is its chartering new waters with embrace of a circular economy doctrine. The objective of which is to improve both the environment and the economy of activities by retaining and recovering all valuable resources in circulation. 
At WWTPs, a clear circular economy business case is evident in the recovery of nutrients, including phosphates from incinerated sludge ash, and phosphates in the form of struvite from wastewater. 

Further still, ash is being recovered and used in construction materials. Already beyond proof of concept, several Danish WWTPs are pioneering resource recovery systems, including Åby WWTP in Aarhus where struvite is turned into ‘PhosphorCare’ fertiliser.

DHI’s Ottosen remarks: “We initiated nutrient removal some years ago and have matured technologies over the last twenty-five years. Our experiences to date mean that in Denmark we have a lot to share about delivering nutrients from waste as commodities, and the path to redefining treatment plants as economically and environmentally productive enterprises.”

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