Beyond water – changing the paradigm of reuse Open Access

Reclaimed wastewater has emerged as a viable option over the past few decades, as major cities are threatened by water scarcity and severe droughts. Early wastewater reuse schemes were mainly a means to avoid costly or unpopular discharge options. In these early schemes, reclaimed water was used mainly for water uses not previously served by town water supplies, such as the irrigation of golf courses and farm forestry.

The objective of many reuse schemes has changed to supplementing potable water supply, primarily by replacing non-potable urban demand. Further, water scarcity pressures are now leading to planning and building a growing number of indirect or direct potable reuse schemes, building on the success of a few well-established long-term potable reuse schemes.

The design and infrastructure of water reuse schemes is currently limited to treating and reclaiming water, making retrofit for the recovery of other potentially valuable by-products prohibitively expensive. In the future, the maximum recovery of other valuable resources needs to be a consideration in the design at the start of any new project. Research needs to find ways for the adaptive change of current technologies to sustainably recover these by-products.

In line with the need for the reduction of greenhouse gas emissions, the emerging focus should be on the recovery of more resources from urban wastewater, including additional water, nutrients, cellulose and volatile fatty acids (VFAs). The by-products of wastewater treatment need to be redefined as valuable resources and these need to be treated and recovered in a more sustainable way.

To date, the normal practice has been to remove the energy potential (biological oxygen demand (BOD)/chemical oxygen demand (COD)), nutrients and other contaminants to meet environmental discharge standards, but few of these valuable resources, other than some of the water and some energy, are normally recovered.

In a conventional treatment, energy is mostly recovered from the anaerobic digestion of approximately 40% of the BOD removed in the primary sedimentation by inefficient combustion of the CH₄ produced.

Other options for greater biochemical energy recovery currently being researched include microbial fuel cells (MFCs), anaerobic membrane bioreactor (AnMBR) treatment technologies, as well as the pyrolysis of sludges.

Phosphorus is removed during coagulation or used as a biological sludge. Biological nitrogen removal occurs by controlling dissolved oxygen in stages of the activated sludge treatment process. Both require significant energy for aeration with associated CO₂ emissions.

Nitrogen compounds are commonly removed by converting them to N₂ gas in an activated sludge treatment plant, but there is a significant potential for the escape of N₂O in the process, which has a greenhouse potential of 265 times that of CO₂.

There is a growing global demand for the production of green hydrogen to substitute for fossil fuels. Even with emerging efficiency improvements in electrolysers, each ton of hydrogen produced needs significantly more than the theoretical 9 tons of water. Reclaimed water is an obvious water source for hydrogen production, which produces O₂ as a ‘by-product’, creating enormous potential for synergies to reduce the energy required for aerobic treatment.

A case study looks at the potential significant theoretical resources available from a typical wastewater treatment plant.

Estimates of the COD for untreated sewage (Metcalf & Eddy 2003) are as follows:

• low 250 mg/L,

• medium 430 mg/L and

• high 800 mg/L.

Xie (2011) estimates that the energy embedded in wastewater COD when oxidised to CO₂ and H₂O is 1.47 × 10⁷ joules per kg of COD. Heidrich et al. (2011) estimated the internal chemical energy of wastewater to be 1.68 × 10⁷ J/m³ for a wastewater mix with domestic wastewater and industrial wastewater and 0.76 × 10⁷ J/m³ for pure domestic wastewater.

Assuming the more conservative estimate of 1.47 × 10⁷ joules per kg of COD and a mid-range of 430 mg/L COD, the energy in urban wastewater is 0.632 × 10⁷ J/m³ or 1.76 kWh/m³.

The estimated 0.15–1.4 kWh/m³ to treat domestic wastewater (NSW Office of Environment & Heritage 2019) is less than the potential biochemical energy. The energy required to treat domestic wastewater for potable reuse is in the range of 1.2–2.1 kW/m³ (Tow et al. 2021), which could largely be achieved without additional energy.

The well-established method for energy recovery from domestic wastewater is the anaerobic digestion of the primary settled sludge and, in some cases, the addition of small quantities of waste-activated sludge. The primary sedimentation process has the potential of capturing up to 40% of incoming BOD or 20% of incoming COD, assuming a BOD/COD of 0.5 (Metcalf & Eddy 2003).

The anaerobic digestion process produces about 65–70% CH₄ and 25–30% CO₂ plus smaller amounts of N₂, H₂, H₂S and H₂O (Metcalf & Eddy 2003). These impurities limit the energy recovery from digestor gas due to inefficient combustion. The energy needed for heating, circulating and sludge drying and handling further reduces the efficiency of the energy balance for this process and, therefore, reduces net energy recovery.

Despite these shortcomings, where energy recovery to produce electricity is installed, it is common for sufficient energy to be recovered to power the whole conventional activated sludge (CAS) wastewater treatment process and in some cases even with some excess power available to the grid.

While this technology is inefficient in recovering substantial energy potential from the wastewater, it also has a number of significant sustainability threats, including the CO₂ released from combustion, plus the potential fugitive leakage of CH₄ and CO₂ to the atmosphere from digestors, dissolved gases in supernatant and further breakdown of the unrecovered residual energy in the digested sludge.

AnMBRs are emerging as potential alternative treatment processes which, among other benefits, have higher energy recovery from COD. Kong et al. (2021) found COD removal rates of 90% and biogas produced from a pilot-scale AnMBR with a methane content of higher than 77%. They found that the AnMBR process represents a significant energy-saving process compared to the CAS process, with a theoretical net energy potential of +0.174 kWh/m³ and a practical net energy potential of −0.014 kWh/m³ compared to −0.195 and −0.421 kWh/m³ for the CAS process.

AnMBR technology can be a sustainable treatment process to be developed to increase energy recovery and reduce input energy requirements, and improve the effluent quality. In addition to the capacity for an AnMBR to remove a broad range of trace organic contaminants, converting organics to biogas, they can also liberate nutrients to soluble forms (e.g. ammonia and phosphorus) for subsequent recovery (Song et al. 2018).

To adapt to the existing plant infrastructure, such as sedimentation tanks for retrofitting, AnMBR technology shows a potential benefit that needs further exploration.

Like chemical or hydrogen fuel cells, the MFC using the microbial metabolism of organic pollutants can directly convert the chemical energy in wastewater into electrical energy while reducing the contaminants.

A membrane separates the two electrodes, into two chambers: (1) the anode chamber, where the bacteria oxidise organic matter in anaerobic conditions, releasing electrons to the anode electrode, protons (H⁺) to the solution, which migrate through a membrane to the cathode, and CO₂ to the atmosphere. (2) In the cathode chamber, oxygen is reduced by the electrons.

While several reported bench and pilot-scale MFCs have been developed, the technology has not yet developed to a plant scale. Many trials have shown that MFCs can remove nearly 70% of COD, but they do not efficiently turn these removed organics into electrical energy (Bird et al. 2022). The release of CO₂ is also a sustainability problem.

The potential for the oxidation of pollutants and reduction of the resulting CO₂ to high-energy compounds, such as CO or CH₄, in one reactor, using multiple photocatalysts, has been demonstrated by Zou et al. (2016) and Zheng et al. (2024). This process replicates the photosynthesis that converts CO₂ to O₂ and organic material.

There is normally a difference in temperature between sewage and the discharge environment, with this differential increasing in cooler climates. The embedded potential energy from this temperature differential in the significant volumes of sewage represents a possible energy source. The theoretical energy from heat differential is the specific thermal capacity of water, 4,180 kJ/m³ per degree C, which has a theoretical energy yield of 1.16 kWh/m³ per degree C of temperature differential (Hao et al. 2019).

This temperature differential can be converted to an electrical current with a thermoelectric generator or Seebeck generator, invented in the early 19th century (Seebeck 1825). While the poor efficiency of conversion limits this option (historically typically 5–8% but possibly higher with modern development), this technology has been used successfully to power long-distance spacecraft such as the Voyager series. With the large volumes of treated water discharged and, in some locations, a significant temperature difference in the winter and/or at night when other renewable sources are less effective, further development of this significant energy source can be considered.

Heat pumps, a well-established and energy-efficient technology, offer a greater short-term potential to more efficiently recover the energy from thermal difference between sewage effluent and the receiving waters and/or surrounding air. This energy could be used for heating or cooling purposes: district heating/cooling, agricultural greenhouses, and even for drying dewatered sludge, recovering some 38% of the theoretical thermal energy (Hao et al. 2019). A limitation is that the energy recovered needs to be used at or near the treatment plant.

The salinity gradient that occurs when an effluent from a wastewater treatment plant discharges to the environment has significant energy potential. For example, approximately 0.75 kWh is dissipated when 1 m³ of freshwater flows into the sea (Helfer et al. 2013). Even with the salt load typical in an ocean discharge plant, there is significant potential to recover some of this energy.

Pressure-retarded osmosis technology, discovered in 1973 in Israel, is one way to recover this potential energy (Helfer et al. 2013). Brines with high concentrations of dissolved salts from membrane treatment have the potential for energy recovery from the significant salinity differential. Treated effluent with low dissolved salts discharged to the sea also has the potential for energy recovery from the salinity differential. Where treated wastewater effluent which has not undergone membrane treatment (and thus with higher total dissolved salt concentration) is discharged to rivers with low salinity, there is also a possible opportunity for some energy recovery. This renewable energy source has the advantage of not relying on weather variables, such as the sun and wind, but it varies in response to changes in the dissolved solids in the inflow to the wastewater treatment plant or to salinity changes when discharging to estuaries or to tidal rivers.

Phosphate fertiliser is sourced from ever-declining reserves of phosphate rock, deposits of bird or bat excreta, with the added energy of extraction, processing and growing transport as sources become scarcer, harder to extract and process, and remote. Global reserves of phosphate will last between 50 and 100 years (Zhang et al. 2022).

We remove these nutrients from our wastewater to meet environmental discharge standards; however, with some limited exceptions, we do not recover them for reuse. Wastewater treatment plants remove 1.3 million tonnes of phosphorus (P), which, if fully recovered, could meet 15–20% of the global demand for P (Zhang et al. 2022).

The total P in wastewater is significantly greater than the measured soluble P in the influent (typically 5–8 mg/L). In the anaerobic zone of a biological phosphorus removal treatment plant, concentrations of up to 40 mg/L can be found due to the release of P from cellular material; this is subsequently taken up in the phosphorous-accumulating organisms in the subsequent anoxic or aerobic zones, which are removed in the sludge (Metcalf & Eddy 2003).

With the main objective of removing P as a pollutant in the final effluent or within organic sludge, removal by coagulation or biological processes has been successful.

If the paradigm shifts to maximising the recovery and reuse of P from the total (soluble and organically bound) P in the inflow, there needs to be a new approach. Recovery from ash or sludge, using acid leaching, from a significant proportion of this organically bound P is feasible (Wang et al 2023); however, the removal at the intermediate stage, where most of the P is in a soluble form, would seem to be a more sustainable option.

The precipitation of P as struvite for use as a fertiliser has been developed as an established technology though not yet widely used. AnMBR technology can maximise soluble P and the chemical precipitation of P, including the organically bound component that is not currently easy to recover.

Other potential recovery technologies, including electrochemical precipitation and the use of biochar for adsorbing P, have shown some potential (Di Capua et al. 2022 and Truong et al. 2023), although not yet at plant-scale application.

It has been estimated (Kehrein et al. 2020) that fertiliser production accounts for more than 1% of the world's greenhouse gas emissions and energy demand with over 90% of these emissions related to the production of ammonium fertiliser. From a sustainability perspective, it is a paradox to produce nitrate fertilisers with high-energy input and then to use large amounts of energy in wastewater treatment plants to destroy and remove the same nitrogen compounds (produced by our wastes) through biological nitrification and denitrification.

Typical concentrations of nitrogen (N) in wastewater have a broad range of 20–85 mg/L, with an average of 40 mg/L (Corbitt 1990) in various forms and oxidation states. The most common method of the removal of N from wastewater is biological nitrification and denitrification in an activated sludge treatment process. Nitrogen is, in theory, released as N₂ gas. In reality, while a significant proportion of the N will be released as N₂ gas, depending on the oxygen available in various stages of the biological process, there may also be significant N₂O released. N₂O is a greenhouse gas with a global warming potential of 273 times that of CO₂. Wastewater treatment is the fourth largest source of N₂O emissions, accounting for between 3 and 5.6% of total global N₂O emissions (Kemmou & Amanatidou 2003).

In addition, the aeration of an activated sludge process in a wastewater plant uses 56% of the total plant energy or about 0.16 kWh/m³ for larger plants (Metcalf & Eddy 2003). The (scope 2) CO₂ e emissions from this energy are substantial (about 0.1 kg CO₂ e/m³ treated) and because of the significant (Scope 1) emissions from the N₂O released, it is not possible under this scenario to achieve net zero emissions even if renewable energy is used.

Recovery of N from the treatment process is still not a mainstream option. One area of potential is anaerobic treatment using AnMBR, which liberates nitrogen (and phosphorus as described above) in a soluble form of ammonium (NH₄⁺), thus facilitating potential high recovery rates through subsequent precipitation. Options include membrane processes, ion exchange, electrodialysis, photosynthetic bioreactor and natural zeolites as an absorbent (Song et al. 2018).

Cellulose, mainly derived from toilet paper, represents about 30% of the total COD of wastewater in some European countries, although the percentage varies significantly in different countries. Cellulose is difficult to treat and becomes a significant component of sludges from wastewater treatment.

The potential for the recovery of a high proportion of the cellulose, particularly if mechanical separation occurs during the primary treatment phase, could meet the demand for industrial uses of the cellulose and reduce the energy demand in the subsequent treatment stages (Wiśniowska & Kowalczyk 2022).

There are a number of potential uses for the recovered cellulose to replace virgin material used in the building industry (Palmieri et al. 2019). As the original material is sourced largely from forest clearing, there is also an obvious potential sustainability benefit to offset the carbon emission associated with this clearing.

VFAs are generated as by-products of the anaerobic sewage treatment process, including acetic acid, propionic acid and butyric acid. VFAs can be used as substrates for many biochemical processes, such as biopolymer production and bioenergy production (Mineo et al. 2024), and can potentially replace currently used non-sustainable sources for these processes.

The treatment of wastewater, using membranes to recover ever-increasing water yields, means that the resulting brines are more concentrated. The potential to recover additional resources and, in particular, metals is enhanced by this concentration. Technologies developed for recovering metals from metalorganic complexes in industrial wastes, such as those from the electroplating industry (Chen et al. 2023), could be applied to recover metals from these brines.

Hydrogen can replace fossil fuels to decarbonise transport and fixed energy production be used in several reduction industrial processes, such as iron and steel production.

Technologies emerge to produce hydrogen directly from organic components in the wastewater, such as microbial electrolysis cells or fermentation processes (Heidrich et al. 2011).

The main synergy with hydrogen production is the availability of a good quality potential watersource reclaimed from wastewater for use in electrolysers. The production of hydrogen by electrolysis requires from a theoretical 9 L to in practice up to 20 L of water for each kg of hydrogen produced, depending on the efficiency of the electrolyser. With growing world water scarcity and competing demands, the use of water reclaimed from sewage has emerged a water source for hydrogen production.

The electrolysis of water to produce hydrogen also produces O₂ as a by-product. The pure O₂ in the aeration process can significantly reduce the energy requirements for the activated sludge treatment replacing air (20% O₂), although there are challenges in achieving efficient and safe O₂ absorption in the biological treatment.

Donald & Love (2023) concluded that producing renewable hydrogen from reused water can displace grid electricity generated from fossil fuels presently used to power wastewater treatment plants (WWTPs).

The heat generated by the electrolysers could possibly be recovered to aid the anaerobic digestion process or recovered as part of the thermal difference of the discharge.

Another possible synergy is the production of aviation and other liquid fuels produced from VFAs with hydrogen. Based on experimental results, Wu et al. (2024) have estimated a favourable cost-benefit and greenhouse gas reduction for several alternative processes.

The resources in a typical wastewater treatment plant have illustrated the opportunities for greater resource recovery. The Wollongong Water Resource Recovery Facility is a coastal treatment plant with a population of 200,000 and an inflow of 49,800 m³/day (Sydney Water 2024).

The plant is a positive example of water reuse and energy recovery. It recycles almost half of the normal inflow with the remaining high-quality effluent discharged to the sea by an ocean outfall. The plant has a 615 kW cogenerator that recovers up to 300 MWh from digestor gas per month or up to 10 MWh/day on average. The treatment consists of primary settling and sludge digestion, activated sludge treatment, filtration and disinfection with additional treatment for three different reuse streams.

To expand resource recovery, the following table summarises the theoretical energy and other resources, available based on publicly available data from Sydney Water (2024).

To achieve the full potential energy from COD, a modified treatment process, such as AnMBR, is required. This process would efficiently convert the organic material currently treated in the secondary treatment process to usable energy, rather than consuming a significant amount of energy to remove these organic pollutants.

Similarly, the recovery of phosphorus and nitrates requires a modification to the treatment process also compatible with the need to recover more energy from the organic load.

Recovery of other forms of potential energy, such as the thermal difference and forward osmotic pressure, will require further development and application of current or emerging technologies into practical and cost-effective ways to recover the significant potential energy.

The reuse of urban wastewater has changed over recent history. Earlier reuse schemes, with some notable exceptions, were mainly planned as a cost-effective and environmentally sound way to avoid discharge into sensitive waterways. This was possible where irrigation was less expensive than the treatment required to meet increasing environmental discharge standards and receiving environmental options. These schemes normally involved new uses for water, such as irrigation of sporting and community facilities or irrigation of pasture, crops or farm forestry as an agricultural resource, replacing dryland farming and grazing.

Increased global water scarcity, accentuated by severe droughts around the world early in the century, has shifted the focus of reuse towards potable replacement for industrial and community facilities. A more recent change is to now regard purified reclaimed water as a product to contribute to the potable supply.

There are already significant benefits of water reuse. There is also an opportunity to reduce the waste of other valuable resources for beneficial reuse. These resources are currently removed but mostly not recovered.

There are international agreements to reduce CO₂ emissions that progress towards net zero. It is, therefore, also essential to reduce the significant emissions from current treatment processes to help meet future global sustainability targets. Currently, only about 20–25% of available biochemical energy is recovered. Greenhouse gas emissions from combustion and fugitive emissions offset much of the sustainability benefits from the energy recovery.

Although more difficult to recover, there are also other significant energy sources:

• additional biochemical energy,

• energy from the reverse osmotic pressure and

• temperature differential of discharge and receiving water.

We need to investigate potential technologies to sustainably recover these valuable and significant energy resources.

Essential agricultural nutrients, such as phosphorus, are becoming rarer globally, and extraction and production are becoming more energy-intensive. At present, there are very limited recovery and reuse opportunities for these nutrients.

Nitrogen fertilisers are largely produced by energy-intensive processes, accounting for more than 1% of global greenhouse gas emissions. Ironically, we use significant energy and add significant greenhouse gas emissions for the removal of the same chemicals. Why don't we recover them for beneficial reuse?

Other resources that are currently removed but not recovered include:

• Cellulose, mainly from toilet paper, is largely sourced from forest clearing, which is a significant contributor to the atmospheric greenhouse gas.

• VFAs can be used in many industrial processes or liquid energy production, such as aviation fuel, replacing non-renewable resources and offsetting the treatment cost of wastewater.

Complementary technologies, such as electrolysis, are used to produce hydrogen, using reclaimed water. The oxygen produced as a by-product can enhance the treatment process and reduce the energy input needed. Hydrogen produced can also be used in combination with VFAs to produce liquid fuels, such as aviation fuel and methanol.

As more water is reused at higher quality standards and higher water yields using membrane treatments, the brines produced become more concentrated. This concentration presents an opportunity to recover some of the resources that are in low concentrations in wastewater but are important to offset mined or manufactured products. Recovery of some metals and other minerals and microplastics may become feasible from these brines with the added benefit of further reducing the disposal costs or environmental impact of the brine.

We need to change our attitude toward the by-products of water reclamation. Future planning and capital outlay must consider the possibility of sustainable recovery of all potential resources from wastewater even if we do not have the technology or social approval to realise those possibilities at present.

When new treatment plants are planned, the capacity for future potential adaptability to new and emerging technologies and resource recovery options should be incorporated. The water industry also needs to explore ways to adapt the existing infrastructure to incorporate new technologies with lower sustainability impact rather than the demolish and rebuild option. It is important for the industry to develop methodologies to assess the economic benefits of retrofitting these new technologies, as opposed to demolishing and rebuilding.

Economics and social impacts and acceptance are important balancing elements of sustainability. It is beyond the scope of this paper to address these important aspects of sustainability in detail, but these should also be considered as part of the ongoing discussion on the issues raised in this paper.

The recovery of additional energy and other resources makes the treatment of wastes and the recovery of water and these other important resources are no longer a net energy user but an energy exporter without net greenhouse gas emissions, which is an important potential economic benefit. The economic and social benefits of avoiding the mining, harvesting or production of these resources which we currently throw away after their removal from our wastes, are also an important consideration.

Technologies developed to achieve this objective must also consider the limitations of poorer and less developed communities to bring them the resource recovery and improve their access to safe drinking water and sanitation. If this can be achieved with a reduction of greenhouse gas emissions and enhanced resource recovery, the economic and social benefits will be substantial.

Reliance on current technologies and simply shifting the significant energy used in these technologies to renewable sources does not make sense in the challenging future. The global water industry must quickly work with the research and development to demonstrate new technologies to sustainably recover most of the other resources and the valuable water from our urban wastewater.

The advice of the Associate Professor Muttucumaru Sivakumar and editorial assistance from Dr Roslyn Muston are greatly appreciated. The work of a number of undergraduate and graduate students has helped to challenge and expand the author's thinking on this topic. These engineers, their peers and colleagues from other disciplines will enhance and realise the concepts in this paper in the future.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.