The metamorphosis of wastewater treatment plants into water resource recovery facilities has sparked fascinating new insights and opportunities. Early on in these emerging WRRFs, the focus was directed to recovering usable water for irrigation purposes and the distribution of usable biosolids as a soil amendment. This low hanging fruit has propelled WWTPs firmly into the minds of the population as a viable solution to dwindling resources.

Energy costs to operate a WWTP are significant. The size of the facility and the use of advanced treatment techniques continue to add to the energy demand. The steady increase of the cost of conventional energy sources both in money and carbon emission concerns have created the drivers to look for sources of renewable and recovered energy to offset the operational demand.

Energy delivered to your door

Tremendous sources of energy get delivered directly to the WWTP by the sewer networks. One of the most potent sources of energy in the arriving wastewater is the biodegradable material that is captured and redirected to anaerobic digesters. The methane gas produced in the digestion process is used in cogeneration to drive microturbines that both generate electricity and harvest thermal energy for building heating and enhancing digestion. Energy creation from anaerobic digestion is so effective that additional sources of FOG are sought out in the surrounding community to radically increase energy production from the digesters. The addition of FOG has enabled an increasing number of WWTPs to reach net-zero energy goals, and more facilities are enabled to be a net producer of renewable energy.

Open for business

The closer we examine what is possible for resource recovery in the WWTP, the more opportunities emerge to generate revenue. More recently the idea of a circular economy has woven its way into the resource recovery discussion. The nexus of the circular economy and the WWTP is being explored. In the Scottish report “Water and the circular economy – Where is the greatest sustainable economic benefit for resource recovery in the water cycle?” analysis has determined six resource areas, of which five have a state of readiness to create value for reclamation in the collected wastewater stream:

• The most significant potential lies in the recovery of energy from raw water and wastewater with heat pumps
• Anaerobic digestion of wastewater can also generate a significant amount of resources, in particular methane, energy and CO2 savings
• Organic solid waste, in addition to wastewater, can supplement anaerobic digestion
• Recovery of biopolymers (polyhydroxyalkanoates, PHAs, polylactic acid, PLA)
• Recovery of inorganic materials

Similarly, Amsterdam in the Netherlands is active with an initiative to recover resources in the context of a circular economy. They make an interesting observation: “The problem is not the availability of technology for resource recovery, but the lack of a planning and design methodology to identify and deploy the most sustainable solutions in a given context.”

In the article “The Risks and Mitigation of Plastics in Wastewater,” it was identified that the WWTP was both the cause and the cure to microplastics entering in the waterways. In that article, it was also pointed out that a majority of the microplastics were captured and removed, mainly in the primary treatment zones via solids skimming and sludge settling processes. In both the Scottish and Dutch reports they targeted the solids settling zone as the key point for extracting additional resources such as biopolymers (PHA) and cellulose. It is an interesting intersection where, at the same point in the treatment process, the offending microplastics are collected in the primary sludge, so too is a source for a biodegradable plastic alternative using PHA. This blend of collection and repurposing is on track with the recommendations made in Part IV of the Ellen MacArthur Foundation report to the 2016 World Economic Forum calling for development and alternative sourcing of biodegradable plastics.

Sustainable solutions

Circling back to part of the quote from the Amsterdam group “…identify and deploy the most sustainable solutions in a given context,” optimizing or modifying a process can potentially reduce an energy load. Microsieving can be used both as an advanced primary technique to harvest and redirect recovered materials, as well as to provide a mechanism to lower energy loads deeper in the process. By applying a process intensification method of using microsieving technology as an advanced primary clarifier, a means of high-rate, controlled carbon diversion is opened up. Diverting carbon in primary sludge to anaerobic digestors can lower (by a third or more) the energy cost for aeration in the activated sludge basins by reducing BOD/COD loading. Microsieving for primary clarification can be controlled to allow for the return of appropriate organic nutrient levels required for biological nutrient reduction techniques to operate properly.

Historically for smaller plants, aerobic digestion processes were a practical design choice to accomplish the facility’s treatment objectives. With increases in energy costs, and rising costs of sludge disposal, alternative processes that reduce the energy required and minimize sludge disposal volumes are being explored. Cost-effective developments in anaerobic digestion technologies have opened the door for practical implementation in smaller facilities. With the addition of anaerobic digestion, sludge diverted from the advanced primary clarifier offsets the aeration cost for the biological oxidation processes, because this sludge has optimized energy potential for efficient and effective anaerobic digestion and subsequent energy generation. In A Guide to Net-Zero Energy Solutions for WRRFs (WE&RF ENER1C12) the authors indicate, “Anaerobic digestion with combined heat and power (CHP) was the most advantageous approach to energy recovery, reducing energy requirements by up to 35% at WRRFs that have anaerobic digestion.” Carbon diversion using chemically enhanced primary treatment or A-stage processes could help plants achieve energy neutrality.

It keeps getting better

As it becomes financially viable there are more resources to be extracted. Municipal wastewater contains an appreciable amount of cellulose (primarily toilet paper). One common use of recycled cellulose is building insulation. Biopolymers can also be harvested for the creation of PHA biodegradable plastics. Practical techniques for struvite (phosphorous) precipitation are being introduced into the market.

The resource recovery facility consistently demonstrates the ability to be a front-runner, leading the way in producing a sustainable example that is in step with the new circular economy. As these WRRFs continue to be created, it is clear that there are opportunities in the capture and harvest of valuable resources that were previously passed on to landfills or into the environment. Further, through process intensification approaches, it also possible to accomplish these identified resource recovery objectives while still lowering the net energy required to operate. This accomplishes not only moving the needle for operating a WWTP from an expense to a revenue source, but it provides another notable contribution to reducing carbon loading in the environment.

The world is looking for courageous people and processes that are part of the solution and not part of the problem. As you go about your daily work in the water profession, you can draw great satisfaction — the treatment plant operator is at the command of a powerful emerging resource generator.