Sequester — redefining wastewater

One of the most encouraging aspects to the Danish water landscape is found in its having fostered some of most compelling displays of sustainable wastewater management in the world. Indeed, already in operation at several wastewater treatment plants (WWTPs) around the country are solutions 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 having 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, to be clear, 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 byproducts for the agricultural industry. Alongside Germany, which borders Denmark to the south, the country is without doubt at the forefront of resource recovery.

Jens Munk-Poulsen, manager of wastewater in Skanderborg Utility, provides outlook on the themes driving innovation across the Danish wastewater scene, saying: “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, again largely achieved through collaboration with DHI.”

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 underestimated 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.”