Adopting the circular model: opportunities and challenges of transforming wastewater treatment plants into resource recovery factories in Saudi Arabia

The grand challenges facing society in the current century are water, energy, food security, and climate change (UNDP 2022). These challenges are exacerbated due to the rapidly growing human population, leading to increased resource (water, energy, food, and chemicals) consumption with a concomitant production of large volumes of ‘used resources’, commonly recognized as waste. Water is the most widely used (100-fold greater) natural resource compared to all other natural resources combined (Rittmann & McCarty 2020). It is required to satisfy the domestic, agricultural, and industrial needs of around 8 billion people on the planet. However, readily available freshwater with adequate quality represents less than 1% of the total water on Earth (Gleick 1993). The ever-growing population, urbanization and economic development are putting unprecedented pressure on finite water resources, especially in arid regions. Around 2 billion people are living in countries or regions with ‘absolute’ water scarcity (<500 m³/year/capita), and two-thirds of the world population could be under ‘stress’ conditions (between 500 and 1,000 m³/year/capita) (FAO 2022b). The environmental services and water-related engineered infrastructure cannot handle the wastewaters of all water users and the discharge of untreated wastewaters generated by human activities contaminates freshwater natural resources.

The situation is even worse in the Gulf region, including Bahrain, Kuwait, Oman, Qatar, the Kingdom of Saudi Arabia (KSA), and the United Arab Emirates (UAE). This region is characterized by scarce rainfall (<100 mm/year), high evaporation rates (>3 K mm/year), and scarcity of renewable water resources (ElNesr et al. 2010; Aleisa & Al-Zubari 2017). The average annual per capita renewable water sources have already reached the chronic water scarcity line (<500 m³/capita/year, Table 1), which would adversely impact socio-economic development (FAO 2022b). The region relies mainly on energy-intensive seawater and brackish water desalination, followed by extraction from non-renewable groundwater resources, to satisfy their demand for water.

KSA, the largest producer of desalinated water, has experienced rapid economic and population growth and development over the past nine decades since oil was discovered. In 1970, KSA had a population of 5.8 million; it increased to 35 million in 2020 and is expected to increase to 40.1 million by 2030 (GAS 2020). Desalination plants will continue to be developed to cater for the growing urban water demand as well as shortages resulting from reduced reliance on ground and surface water. In 2026, the existing and committed desalinated water supply is expected to provide 2.18 billion m³/year (SWPC 2020). On the demand side, KSA is among the world's top five consumers of water on a per capita basis currently standing at 360 L/day, which is significantly higher than other developed countries (SWPC 2020). Total water demand is expected to increase to 14.5 million m³/day by 2026 with a gap of 4.5 million m³/day estimated in 2026 at the national level. Out of this national water gap, a 4.1 million m³/day gap is expected to be served through newly constructed or planned seawater reverse osmosis (SWRO) mainly due to its economic advantages (IDA 2020). SWRO requires 3–8 kWh/m³ of desalinated water for large-to-medium-size plants, respectively (Al-Karaghouli & Kazmerski 2013). For small-size plants, it can be as high as 15 kWh/m³ (Al-Karaghouli & Kazmerski 2013). Assuming an average energy consumption of 6 kWh/m³, more than 15 billion kWh energy is used by desalination plants in KSA. Currently, KSA desalination plants emit on average 2.5 kg CO₂/m³ of desalinated water produced (Reddy & Ghaffour 2007), which corresponds to 5 million tonnes of CO₂/year through desalination plants (producing about 2 billion m³ water/year) only.

Given the stress on water resources in this region and the number of energy resources allocated to desalination and their carbon footprint, it is imperative to maximize water-use efficiency. In fact, there is an emerging interest worldwide in resource recovery from waste streams towards a circular economy. In this regard, wastewater (i.e., used water) is no longer viewed as a waste material to treat with energy expenditure, but rather as a valuable resource of water, energy, nutrients (nitrogen and phosphorous), and materials (e.g., bioplastics, cellulose fibres, alginate, and metals). Most importantly, the reuse of treated water is becoming mandatory to fully address the water scarcity issue in KSA and many water-scarce countries around the world. Transforming wastewater treatment plants (WWTPs) into resource recovery factories (RRFs) is currently a key driver for research and development of next-generation wastewater treatment technologies. This article outlines the water challenges that are arising in KSA and the major water research programmes/themes undertaken to address these major challenges.

Wastewater remains a largely untapped resource for the circular economy. For example, valuable substances in wastewater include water, phosphate, and ammonium that can be used as recovered fertilizer, lipids as biofuel, and cellulose for bio-chemicals and bio-composites. In addition, wastewater provides a source to produce biodegradable bioplastics (i.e., polyhydroxyalkanoates (PHA)), a possible replacement for the petroleum-based plastics that biodegrade very slowly and affect marine life. As a result, these carbon-based elements are no longer directly converted to CO₂ and/or CH₄ as is happening in the current practice of wastewater treatment but are converted to valuable products leading to a decrease in GHG emissions. Smarter use of secondary raw materials will create a safe and sustainable supply of raw materials to the industry. Smarter use of resources from wastewater will also help protect the environment and at the same time preserve essential resources for current and future generations.

The circular economy model also reduces threats to future growth that come from unsustainable patterns of resource use. It takes account of the depletion and waste of natural resources as a loss to the economy, which is not captured in existing GDP measures. According to a study, a circular economy model could generate up to US$138 billion (Saudi Riyal 518 billion) in economic benefits for the Gulf Cooperation Council (GCC) between 2020 and 2030 (Bejjani et al. 2018). These benefits would largely arise from enhanced competitiveness, a lowering of environmental pressures and costs, making raw material supply chains more secure, boosting industry innovation, and generating new jobs.

Desalination is the lifeline of KSA, and it provides a significant amount of fresh water to meet national water demand. There is always a desire to develop innovative desalination technologies which require less energy input and contribute less to global warming. There is rapid growth in KSA desalination capacity, and six new installations based on the reverse osmosis (RO) process will further add 3 M m³/day capacity (Narayanan 2022). The desalination demand can be reduced by using impaired quality water sources, such as seawater, for toilet flushing and treated wastewater for non-potable reuse (discussed in Sections 4 and 6). The main challenge associated with desalination is better management of brine discharge, which is especially critical when desalinating saline groundwater in inland areas. However, RO is still facing challenges, such as limited water recovery and biofouling, especially in areas where feed water has higher salinity and nutrient concentrations such as the Gulf water, further increasing the already intensive energy requirements. An assessment of the concentrations and fate of trace elements during saline water treatment will enable us to evaluate the potential for the development of directed strategies for brine mining combined with zero liquid discharge (ZLD). The current research programme on desalination aims to enhance the total water recovery and reduce energy and chemical consumption by:

• Optimizing the RO process through the development of performance control and prediction tools for efficient membrane cleaning and an accurate estimation of the osmotic pressure throughout the pressure vessel to minimize the energy input and reduce (bio)fouling (Desmond et al. 2022).

• Integrating RO with solar energy (Bellini 2022).

• Treating the RO brine using novel direct contact evaporation and condensation (DCSEC) integrated with membrane distillation (MD) processes (Chen et al. 2021). Both DCSEC and MD operate at moderate temperatures (∼50–70 °C) which makes them suitable for the use of solar energy. The whole hybrid process is targeting close to ZLD. Both RO and the proposed novel DCSEC-MD processes will play an important role in future desalination installations due to their flexible scalability from very small-to-extra-large capacities, which will have a direct socio-economic impact on KSA.

The aerobic granular sludge (AGS) process has been shown to be a robust technology for the treatment of saline wastewaters having a salinity of 1% (Wang et al. 2017, 2018). Similarly, marine anaerobic ammonium oxidation (anammox) bacteria, which have intrinsic tolerance to salinity compared to freshwater anammox bacteria, enriched in a membrane bioreactor (MBR) showed good removal of ammonium (nitrogen removal rate (NRR) of 0.27±0.01 kg-N/m³/day and ∼92% N-removal efficiency) from saline wastewater with a salinity of 1.2% (Ali et al. 2020a, 2019a, 2020b). These findings have significant implications for applying AGS or marine anammox process for treating saline wastewater generated from toilet flushing practices. It is anticipated that more cities will adopt seawater toilet flushing practices to relieve the stress of the ever-increasing demand for freshwater (Yang et al. 2015). Furthermore, the problems associated with the corrosion of pipelines and equipment induced by seawater toilet flushing practices can now be solved or mitigated satisfactorily with the experience gained from previous practices (Tang et al. 2007).

Regarding the recovery of resources from sewage, there are different options, paths, and priorities. The current focus is on biogas, cellulose, bioplastics, phosphate, and alginate-like exopolymers (also known as bio-ALE). There are currently 91 medium- to large-scale centralized WWTPs in KSA treating 1.7 billion m³/year of wastewater (Table 2). The creation of new RRF is capital intensive compared to upgrading existing WWTPs to RRFs; and some of the WWTPs in KSA are quite big, especially in big cities such as Riyadh, Jeddah, and Mecca. Economies of scale can play an important role in the cost of recovered resources. This implies that RRFs will probably be created in big cities first that are served with large-scale centralized WWTPs. When experience has been gained and the costs of installations have dropped, it is expected that RRFs will also be built in smaller cities.

The most important resource in wastewater is water, once it has been adequately treated to prevent harm to the consumer (Rittmann & McCarty 2020). KSA uses desalination, an energy-intensive process, with average energy consumption of 6 kWh/m³ (Reddy & Ghaffour 2007; Shahzad et al. 2017), to produce freshwater to meet its municipal water demand. The used water is then treated using aerobic wastewater treatment processes such as the activated sludge (AS) process where ∼50% of the total energy (0.6 kWh/m³) (Bengtsson et al. 2019) is consumed for aeration and most of the treated effluent is released back to the sea without reuse. The need for energy for wastewater treatment might drop significantly due to the introduction of more energy-efficient technologies, e.g., AGS (Pronk et al. 2015; Ali et al. 2019b). This linear water cycle should be changed to a circular water cycle, where the treated effluent should be reused for beneficial purposes such as agriculture and irrigation, groundwater replenishment, industrial processes, and environmental restoration. The average wastewater generated in KSA is estimated at 212.5 L/capita/day (SWPC 2020), roughly generating 2.7 billion m³/year of wastewater. The Saudi National Water Strategy (NWS) highlights the need for treated sewage effluent reuse, which was 17% in 2016 and is set to reach 70% in 2030 (SWPC 2020). An increase in reuse will reduce the pressure on non-renewable water resources and decrease dependency on energy-intensive desalination processes. However, water reuse will not be able to cover the demand for agriculture and other sectors, so KSA will still rely on desalination, but future desalination plants will be coupled with solar to reduce CO₂ emissions and energy consumption.

The pilot testing of the AGS-GDM unit has now been successfully completed with real wastewater. The AGS-GDM system produced superior effluent quality to the neighbouring full-scale AS-MBR with higher concentrations of NO₃⁻-N and PO₄³-P (data unpublished). Currently, the AGS-GDM process is being scaled up under the KAUST Research Translation Program (Near Term Grand Challenge) to specifically tackle the above grand challenges. Under this project, the AGS-GDM unit for wastewater treatment and recycling will be built and demonstrated at a real-world application scale. Also, it addresses many critical aspects, including infrastructure development, water savings, economy, job growth, and impact on the people of KSA. Other technologies that integrate the production of reusable water with energy recovery are discussed in the next section.

Annually, the 91 centralized WWTPs in KSA consume about 1 million MWh—assuming an average-size WWTP requires 0.6 kWh/m³ of wastewater (Bengtsson et al. 2019). These WWTPs have the potential for biogas production of about 112 million m³/year which can cover almost 40% of the energy consumption of the water authorities (Wan et al. 2016; Bengtsson et al. 2019). It is estimated that biogas production at 112 million m³/year leads to a total saving of US$25 million/year (van Leeuwen et al. 2018). It will also lead to a reduction in the use of fossil fuels to drive WWTPs.

The current research programme on wastewater treatment aims to achieve a circular economy in wastewater treatment by integrating energy production and resource recovery (energy) into the production of clean water. In this regard, anaerobic microbial technologies hold great promise for resource recovery (energy) from wastewater. As such, microbial processes based on methanogenesis or electrogenesis offer an opportunity to biologically treat the organic carbon in wastewater with the concomitant recovery of energy as methane (methanogenesis) in AD or hydrogen in microbial electrolysis cell (MEC).

Roughly, 500 tonnes of dry waste sludge is produced every day in KSA (Table 2). Off-site landfilling is the current practice for waste sludge management. Waste sludge generated from WWTPs is being collected and transported to the landfilling sites. Trucking waste sludge is costly (each trip costs around US$ 50–75), and results in traffic congestion, air pollution, and GHG emissions (methane) from landfills. Instead, the waste sludge can be used to recover resources such as cellulose, bioplastics, phosphate, and bio-ALE and later sludge can still be digested to produce biogas. Currently, in the KSA, there is only one anaerobic digester operational in a WWTP in Riyadh. The produced biogas (a mixture of CH₄ and CO₂), after desulphurization, is either flared or used for electricity generators in emergencies.

It is pertinent to mention that the conventional AD process often suffers from operational instability and low biogas yield, caused by substrate characteristics, short-chain fatty acids (SCFAs) accumulation, and/or ammonium ions (Hobbs et al. 2018). MECs could potentially be integrated with existing AD processes to enhance stability and facilitate SCFAs degradation (Hari et al. 2016a, 2016b). The MEC-assisted AD (MEC-AD) process successfully avoided SCFA accumulation during waste-activated sludge (WAS) degradation and suggested that exoelectrogens were much more efficient in SCFA degradation (Sun et al. 2014). Another advantage of integrating MEC with AD is the enhancement of CH₄ production and purity (Feng et al. 2015a, 2015b).

Alternatively, waste sludge production can be minimized by directly cleaning wastewater using AnMBR. Over a course of a 30-week operation, we have observed that our demo-scale AnMBR generates at least 20 times lower volume of sludge compared to the aerobic wastewater treatment process that is operated at the same site and receiving the same type of influent. The AnMBR minimizes the volume of sludge that needs to be disposed of or digested, in turn bypassing the problems mentioned earlier. Instead, energy is directly obtained by converting the organic carbon in the wastewater to methane. In the demonstration-scale wastewater treatment plant (50 m³/day capacity), the AnMBR functions as the core technology for carbon removal, while UV/hydrogen peroxide is used for disinfection and further degradation of organic micropollutants (Augsburger et al. 2021). As AnMBR cannot remove ammonium and phosphate, these nutrients can either be captured first by biochar to convert into solid fertilizers (see section 6.3) or directly reused for urban farming or green landscapes grown in a controlled system (e.g., blue-green walls, (Prodanovic et al. 2017, 2019, 2020)). The nutrients and CO₂ within the biogas may also be suitable to sustain microalgae farms to derive value-added products such as bioplastics, lipids, biomass-based fuels, etc. In a proof-of-concept study, mixed microalgae-bacteria photobioreactors were successfully operated to utilize nutrients that remained in effluents from AnMBR and CO₂ from the biogas (Xiong et al. 2018), in turn enhancing the purity of the effluent stream and the produced CH₄.

Nitrogen is generally present in the form of ammonium or organic nitrogen in wastewater. In KSA, about 75 tonnes of ammonium is wasted every year in the WWTPs (Table 2). Important to realize is that, in order to feed the world's growing population, N₂ in the atmosphere is converted into ammonium for use as a crop fertilizer using the Haber-Bosch process, which is an energy-intensive process (12.1 kWh/kg NH₃-N), consumes about 7% of the world's natural gas, and is one of the fossil fuels causing climate change (McCarty et al. 2011). The ammonium in municipal wastewater comes from the food we eat, and conventional wastewater treatment for nitrogen removal based on nitrification and denitrification generally converts the ammonium back into N₂ gas at an energy expenditure of 4.6 kWh/kg-N (Cruz et al. 2019). The current routes for ammonium removal not only neglect the energy embedded (about 12 kWh/kg NH₃-N) in ammonium (Trimmer et al. 2017) but can also produce N₂O, which is a potent GHG with a 300-fold stronger greenhouse effect than CO₂ and is one of the most dominant ozone-depleting gases (Ali et al. 2016). Therefore, there is a need to develop alternative NH₄⁺ management approaches that centre on the recovery of energy from NH₄⁺ in domestic wastewater rather than dealing with its ‘destruction’ into dinitrogen (N₂) (Cruz et al. 2019).

Phosphate (PO₄³⁻) is a scarce resource on the global scale with limited reserves, especially found in Morocco and China today; almost all the phosphate is mined and used in agriculture. Most of that phosphate ends up in waterways due to run-off and wastewater discharges (Rittmann & McCarty 2020). Phosphate causes eutrophication of surface waters (Morée et al. 2013; Rittmann & McCarty 2020). Like ammonium, phosphate in wastewater must be treated to very low concentrations (<1 mg/L) before being discharged to aquatic environments or when treated water is recycled for non-potable reuse. Currently, enhanced biological phosphorous removal (EBPR) is considered the main biological process for the removal of phosphate from wastewater (Dueholm et al. 2022). Instead of biological removal through the enhanced biological phosphorus removal (EBPR) process, phosphate can be recovered as, e.g., struvite (MgNH₄PO₄·6H₂O). In this way, phosphate becomes available again since the recovered struvite can be applied as a fertilizer. Struvite is formed in digested sludge based on a process with the addition of magnesium chloride (MgCl₂). Raw domestic wastewater contains approximately 10 mg-P/L/person/year (Chen et al. 2020), which corresponds to approximately 17,000 tonnes of phosphate/year in KSA (Table 2). Recently, a phosphate recovery facility was introduced in a WWTP in Amsterdam and other communities, which can produce 900 tonnes of struvite (van der Hoek et al. 2015, 2016). The plant treats approximately 2,000 m³ sludge/day, while the expected savings are €400,000/year (van der Hoek et al. 2016). The savings of this process consist of selling the struvite, lowering the sludge handling costs, and better sludge dewatering. Extrapolated for KSA, the recovery of phosphate could generate approximately US$11.2 million/year from the year 2030 (Table 3).

Cellulose is a material which ends up in sewage due to the use of toilet paper, and cellulose can be recycled from toilet paper (Ruiken et al. 2013). This recovered cellulose can be used in road construction, but further markets need to be developed (Crutchik et al. 2018). A person uses an average of 10 kg of tissue paper/year (Ruiken et al. 2013), which means that the total potential volume is approximately 420,000 tonnes/year in KSA. Nevertheless, not all tissue paper can be recovered at WWTPs. It is estimated that about 25% of cellulose can be recovered with the introduction of fine-sieving at WWTPs (Roest 2017), which will lead to approximately 100,000 tonnes/year. Although there are currently no revenues from cellulose recovery at all, as considerable investments are necessary, the potential revenue is interesting due to the expected price of this material, i.e., approximately US$300/tonne (van Leeuwen et al. 2018).

In the context of digesting cellulose to produce methane, the fermentation process suffers from low yields, and instability caused by the accumulation of acids from the degradation of complex substrates such as cellulose (Lusk et al. 2018). Coupling fermentation with MECs could be a viable approach for efficient biogas production from renewable biomass (Lu & Ren 2016). Several studies reported the integration of MEC with fermentation for cellulose waste degradation (Lalaurette et al. 2009; Wang et al. 2011; Lusk et al. 2018).

Alginate is a naturally occurring anionic polymer. It is a polysaccharide, typically obtained from brown seaweed and has been extensively investigated and used for many biomedical applications which include wound healing, drug delivery, and tissue engineering due to its biocompatibility, low toxicity, and relatively low cost (Roest et al. 2015). Bio-ALE is an alginate-like polymer of sugars and proteins and can be used in agriculture and horticulture, paper industry, medical, and construction industries. Bio-ALE can be applied as fibres, gel, or foam. Fibres can be used for the production of absorbing tissues, gels can be used as glue for the production of fertilizer pellets, and foam can be used for the production of fire-resistant boards. The liquid version can be used for thickening inks, improving paper quality, and increasing the quality of curing concrete (Roest et al. 2015). The first bio-ALE production installations were built in Zutphen and Epe in 2019 in the Netherlands. In a field test, it was demonstrated that 0.2 kg bio-ALE can be produced per kg of Nereda AGS (HaskoningDHV 2017) and a tonne of ALE has market values of approximately US$ 2,000 (van Leeuwen et al. 2018). The extraction of bio-ALE from AGS can be considered up-cycling because a more valuable product is produced from waste. Recently, the National Water Company (NWC) tender a Nereda plant of 50,000 m³/day capacity in Jeddah. It is expected that more WWTPs in KSA will be retrofitted or upgraded to Nereda technology by 2030. Assuming the total capacity of Nereda plants would be 150,000 m³/day by 2030, it would potentially produce 730 tonnes bio-ALE (from 3,650 tonnes of excess sludge)/year. The sale of bio-ALE would generate revenue of about US$1.46 million/year by 2030 (Table 3).

Bioplastic can be produced from sewage sludge as a form of up-cycling. Bioplastics are formed using a complicated process during which volatile fatty acids (VFAs) are produced first and then later fed to microbes which produce the building blocks for bioplastics (i.e., PHA). There have been some experiments and tests that showed an average extraction amount of 30 g-PHA/100 g of biomass was achieved with 98% purity (de Souza Reis et al. 2020). The production cost of this material, however, is currently still rather high, and it is twice as much as the regular market price. Furthermore, there is no available stable industrial production process yet (van Leeuwen et al. 2018). Currently, plastics are produced through petroleum products made from fossil fuels. In future, there will be more demand for bioplastics to minimize reliance on fossil fuel-based plastics.

The existing centralized wastewater-related infrastructure is not able to be modified easily. The creation of new RRFs from scratch is capital intensive compared to upgrading existing centralized WWTPs to RRFs. Therefore, upgrading or retrofitting existing centralized facilities would be the most feasible route at the beginning to achieve a circular economy in the water sector. Once critical knowledge and experience have been gained, resource recovery technologies would also be implemented for newer and smaller-scale installations.

Water reuse should be preferred to achieve circularity. Currently, a centralized approach is used for wastewater management, collection, and treatment. Upgrading centralized WWTPs with water reclamation and reuse facilities could support the increase of water reuse in KSA. However, many centralized WWTPs are located far away from city centres and in areas where water demand for treated effluent is low. In such cases, physical infrastructure (such as pipelines) is needed to transport the treated effluent to areas with high water demand. There is a lack of centralized infrastructure for water reuse and building centralized water reuse infrastructure could be capital intensive. Therefore, modular, adaptive, and/or decentralized approaches should be implemented using sustainable technologies such as AGS-GDM, AnEMBR, and AnMBR, which could enable treated water reuse at a neighbourhood scale. Implementation of a modular, adaptive, and/or decentralized approach for newer installations would certainly promote water reuse and help to achieve the 2030 water reuse target (70%) by the Kingdom of Saudi Arabia.

There are already established technologies for the recovery of CH₄, phosphate, cellulose, and bio-ALE from sludge, which could be easily integrated into existing installations. As mentioned above, conventional AD or modified MEC-AD could be easily integrated into existing facilities to recover CH₄ from waste sludge. However, it should be highlighted that there is no infrastructure available for CH₄ distribution and it could be expensive to build such infrastructure. Therefore, the generated CH₄ should rather be utilized to offset the energy requirement of the wastewater treatment to make the system more energy-efficient or energy-neutral.

This work was supported by the Center Competitive Funding Program (FCC/1/1971-33-01, FCC/1/1971-44-01), Near Term Grand Challenge Funding Program (REI/1/4254-01-01), Research Translation Funding Program (REI/1/4223-01-01), and Impact Acceleration Funding Program (REI/1/4224-01-01) from King Abdullah University of Science and Technology.

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

The authors declare there is no conflict.