Non-biological methods for phosphorus and nitrogen removal from wastewater: A gap analysis of reinvented-toilet technologies with respect to ISO 30500

Mature and emerging technologies for non-biological nutrient removal

Several non-biological technologies are commonly applied at municipal- and industrial-scale WWTPs for N and P removal. However, very few of these technologies have been employed at the household/community scale where NSSS are most relevant. Some methods (e.g. air stripping; breakpoint chlorination; chemical precipitation) are well-established but may be applicable only to one specific target nutrient, while others (e.g. hydrogel/polymer matrix encapsulation; ion-exchange materials; membrane-based separations) are in various stages of development and use depending on the nutrient targeted for removal/recovery. In this section, we give a brief overview of these mature and emerging technologies for N and P removal, while providing commentary on the relevant benefits and knowledge gaps as relates to their application in NSSS. A summary of the limitations and potential R&D opportunities for these technologies is presented in Table 1. It is important to note that the ISO 30500 standard is written with specific guidance (including liquid effluent reduction threshold values) for removal of N and P. While recovery and reuse of nutrients are critical topics for sustainability and implementation of NSSS technologies, the primary technological hurdle currently facing RT system deployment and water reuse is nutrient removal. We briefly mention situations where recovery/reuse is possible and indicate this also in Table 1, as we feel this is an important and exciting area for continued research. However, a comprehensive review of nutrient recovery/reuse is outside the scope of this Open Letter.

Air stripping

Air stripping is a process which relies on the aqueous equilibrium between the ammonium ion (NH₄⁺) and free ammonia (NH₃). Under optimal conditions, this equilibrium is shifted to favor free ammonia which can leave the solution via evaporation. As a physical process, air stripping efficiency is affected by the ambient temperature and pressure. However, the solution pH is the predominant factor in determining availability of free NH₃; higher pH heavily favors NH₃, with the optimal pH for air stripping being ≥10. Optimized NH₃ removal via air stripping is in excess of 90%^3,20,21.

Chemical addition of large quantities of base is required to bring the wastewater pH into the optimal range for air stripping. It follows that acid addition is often required to bring the pH back down to the acceptable range for subsequent effluent discharge. Significant investments in infrastructure and/or land can be required for stripping towers, aeration ponds, etc²⁰. While it may be possible to effectively scale down the infrastructure, conventional ammonia air stripping is largely incompatible with RTTC guidelines due to the need for caustic chemical additives.

Electrochemical stripping may be one viable alternative to conventional air stripping for decentralized systems where intermittent electricity or photovoltaics are available²². In electrochemical stripping, base production is accomplished in situ using electricity and an appropriate anode material, and ammonia is separated from the solution via a gas-permeable membrane and without the need for a stripping tower. This emerging technology faces several challenges, including electrode poisoning/stability, membrane fouling, and generation of undesirable byproducts. These issues will be discussed in more detail in the subsections below.

Breakpoint chlorination

For ammonia-containing wastewater, breakpoint chlorination describes the chemical process whereby a sufficient amount of hypochlorous acid is present to completely oxidize ammonia to nitrogen gas:

The above description and equation are deceptively simplified, as multiple intermediate steps involving the formation and subsequent reaction of chloramines are not explicitly shown. Rather than adding chlorine directly to wastewater, hypochlorous acid can be generated in situ electrochemically by catalytic oxidation of chloride ions at an appropriate electrode surface. Breakpoint chlorination can reliably give ammonia-to-nitrogen conversions of ≥95% with no measurable nitrous oxide formation under appropriate operating conditions^21,23

Breakpoint chlorination initially appears to be an attractive method of N removal, as it typically requires no chemical additives (unless the chloride concentration in the wastewater influent is too low, in which case salt, e.g. sodium chloride, addition is warranted), generates harmless nitrogen gas, and uses technology already deployed in many NSSS. (Electrochemical oxidation of chloride for liquid disinfection is currently used in several RT systems^14–16.) However, there are some important limitations, including the possibility for formation of undesirable oxidation byproducts, many of which are toxic or carcinogenic^24–26. The formation of undesired byproducts has been well documented for electrochemical wastewater treatment processes, including chlorine disinfection^20,27,28. Breakpoint chlorination for ammonia removal uses the same active compounds as chlorine disinfection, but at far higher concentrations, which dramatically increases the chances for formation of harmful byproducts. In fact, the chloramine reaction intermediates themselves are considered undesired byproducts^3,20,21. This makes incomplete oxidation of ammonia to nitrogen problematic, which drives demand for an excess of reactive free chlorine, and necessitates a downstream dechlorination process. Control and remediation of byproducts downstream of the breakpoint chlorination process is thus required, which adds complexity and cost^3,20,27.

Additional issues with breakpoint chlorination include: (1) acidification of the treated effluent and subsequent creation of nitrogen trichloride; (2) need for chemical addition to neutralize pH, causing large increases in total dissolved solids in the treated effluent; (3) finite lifetimes of electrode materials (which contain expensive noble metals and metal oxides) due to surface inactivation/poisoning; (4) need for automated monitoring and control of pH, ammonia, and free chlorine levels; (5) practicality primarily as a polishing technique, rather than for removing high levels of N; (6) does not allow for recovery of N in a bioavailable form^3,20,21. Despite the reliability and efficiency of breakpoint chlorination, the associated downstream problems should exclude its use in the RTTC portfolio. There may be other electrochemical processes for N removal that are less problematic and more effective, e.g. electrodialysis, which will be discussed in more detail below.

Chemical precipitation

Chemical precipitation involves the addition of soluble salts to nutrient-rich wastewater to induce formation of nutrient-containing, insoluble compounds, which are removed by settling (gravity) or filtration. Chemical precipitation is currently the primary method of P remediation in wastewater treatment applications, and the most common compounds used for P removal contain Fe, Al, Ca, and/or Mg²⁹. Aside from the obvious requirement for chemical additives, a potential drawback of chemical precipitation is that P and/or N is merely sequestered rather than being removed from the wastewater treatment system. A consequence of this is the generation of large quantities of sludge that still require disposal and/or subsequent treatment. The sludge quantity is further increased by the need for chemical additives in excess of the theoretical stoichiometric requirements, presumably due to various side reactions^2,20. Small-scale NSSS may lack the space and/or infrastructure required to handle large amounts of sludge on site.

The particular chemical additive to be used in any treatment system needs to be chosen with care, as problems such as scaling and slow precipitation kinetics will be heavily influenced by the influent water chemistry. Often, pH adjustment by addition of hydroxide salts is required to favor formation of the desired precipitate²⁰. The choice of cation used for precipitation is critical and affects myriad factors including overall cost, P removal efficacy, optimal pH and temperature required for P removal, and reusability/bioavailability of the P-containing precipitate³⁰. Resulting trade-offs need to be carefully considered depending on the context and effluent requirements. For example, the Fe and Al salts used for P precipitation are typically much less expensive than Ca and Mg salts; however, Fe and Al phosphate precipitates are not suitable for direct use as fertilizers due to low P bioavailability³¹. On the other hand, using Mg for struvite precipitation can enable simultaneous removal of phosphate and ammonium in a 1:1 stoichiometric ratio and has been shown to be widely bioavailable;³⁰ however, struvite precipitation has a narrow window of optimal pH and cannot be relied upon as the sole method of N removal due to the much higher concentrations of ammonia/ammonium relative to phosphate³². Furthermore, more research is required to address concerns regarding the presence of contaminants in precipitates where fertilizer use is intended^30–32.

There still exists some room for improvement of chemical precipitation technologies, e.g. with the novel application of materials such as calcium silicate hydrate, which can initiate precipitation and simultaneously adjust pH³³, thus decreasing the quantity of added chemicals needed for phosphate removal. Electrochemically-induced coagulation or precipitation using sacrificial anodes can provide better control of critical process parameters, including pH and metal ion dosing, and may be easier to implement than direct salt addition in decentralized treatment settings^34–36. If the nutrient-containing precipitate is a valuable commodity (e.g. fertilizer) and can be efficiently separated from sludge, the use of a chemical additive may be justified by the benefits endowed from nutrient recovery. However, converting chemically-bonded nutrients in some precipitates to a bioavailable form can be difficult². New nanocomposite materials could be designed to increase capacity and regeneration capability³⁷. Depending on the influent characteristics, some industrial byproducts (e.g. gypsum, fly ash, slag) may provide a cost-effective solution for nutrient remediation²⁹. Ultimately, the overall cost, availability, sludge disposal, and potential downstream environmental impact of chemical precipitation processes will need to be evaluated on a case-by-case basis for each wastewater stream and its corresponding effluent requirements.

Hydrogel/polymer matrix encapsulation

Hydrogels and polymer matrices are a relatively new class of low-cost adsorbent materials being studied for P and N capture^38–42. Hydrogels have a high water content and porous structure that facilitates solute diffusion⁴³. As these materials are synthetic, they offer the opportunity to tune the absorptive selectivity and capacity by altering the polymer chemistry. Hybrid hydrogels can also be created by embedding inorganic particles into the polymer matrix, which can further increase the adsorption capacity⁴³. Hydrogels also have high potential for recycling and regeneration^43,44. To date, research for wastewater treatment with hydrogels has primarily focused on adsorption of organic compounds, radionuclides, and dyes^43,44. Recently, a bentonite-based hybrid hydrogel showed selectivity for phosphate adsorption in a mixed-anion waste stream with 99% phosphate removal under optimal conditions³⁸. A commercially available polymer hydrogel was demonstrated to have ammonia removal capacity up to 80% at pH 5.0–8.0, and showed minimal performance loss after regeneration with mild acid washing⁴². Development of new, tailored hydrogels for P and N removal is a promising area for future research.

Ion-exchange materials

Ion-exchange materials are highly porous structures loaded with ions which are selectively displaced by target ions (e.g. ammonium; phosphate) in the wastewater stream. Ion-exchange materials offer several benefits when compared to conventional chemical precipitation methods. For example, while chemical precipitation can handle higher nutrient concentrations, the kinetics of ion-exchange processes are much faster in comparison and thus the removal efficiency is not highly dependent on retention time. Additionally, nutrients captured by ion exchange are more easily recovered in comparison to e.g. chemical precipitation, as ion-exchange processes are reversible²⁹.

A fundamental limitation of ion-exchange materials is that their capacity is inherently limited by the accessible surface area and ion loading of the material; at some point, the adsorbent will need to be replaced or regenerated. Synthetic ion-exchange resins, while potentially engineered to have better selectivity, adsorption and regeneration capacities, and durability than natural zeolites, can be far more costly⁴⁵. On the other hand, certain natural zeolites may be geographically scarce. Conventional methods of material regeneration require large volumes of water and quantities of salts, including caustic acids and bases⁴⁶. There is also the issue of the so-called “paradox of P sorption materials”, wherein the materials that are highly effective at P removal generally show poor performance for water transport²⁹.

Despite these issues, there is opportunity for using ion-exchange materials in a RTTC context. In particular, the option to regenerate the adsorbent material and recover a nutrient-rich solution is attractive when considering strategies for integrating NSSS into a circular N and/or P economy. For any particular ion-exchange material, testing with the real, intended wastewater stream is critical, as influent properties such as pH and organic loading can have dramatic effects on adsorption and regeneration capacities^9,45. Offsite zeolite regeneration using a spoke-hub model could create local employment opportunities, as the regeneration/recovery process need not be highly technical and the regeneration solution can be reused many times. However, access to and cost of salts required for regeneration would still need to be considered⁴⁵. Optimizing the zeolite regeneration process to operate on a small scale with limited additives would be a significant achievement that merits investigation⁴⁶.

Even without regeneration, the low cost of certain natural zeolites and other minerals may make them effective as consumable filtration media in small-scale systems. For example, clinoptilolite is one of the best natural ion exchange materials for ammonium ions (capacity = 2–30 mg NH₄⁺ g^-1), and has been demonstrated to have a lower material cost ($ per g of N removed) than conventional biological nitrogen removal, even when treated as a one-time-use consumable⁴⁵. Spent natural zeolites may have additional value as soil conditioners and fertilizers^46–48. Polonite^® is a particularly attractive material for P removal and recovery due to its high P capacity (up to 12% by weight)^49–51, and reasonable price (< US$1 kg^-1). The exhausted filter media can be applied directly to soil for agricultural use as a slow-P-releasing fertilizer and soil conditioner⁵². If the two processes are used in series, the more basic Polonite^®-filtered effluent could counteract the undesired pH drop induced by in situ chlorine disinfection. We are currently evaluating the use of Polonite^® and clinoptilolite for ISO 30500 compliance in NSSS.

Ammonia removal from wastewater using natural⁵³ and synthetic^9,48 ion-exchange materials has been studied for several decades; for the natural zeolite clinoptilolite, the average ammonia removal efficiency is ≥95%²¹. Despite the use of ion-exchange materials for decades in remediation of ammonia, ion-exchange materials can still be considered an “emerging technology” for P removal in wastewater treatment applications. Hybrid anion-exchange resins (HAIX) consist of iron (hydr)oxide particles embedded within anion-exchange polymers and have been tested with a variety of wastewater streams, including source-separated urine^54,55. The operational capacity of HAIX media depends on both the influent P concentration and the target effluent P concentration^56,55, and thus use of HAIX in NSSS will need to be assessed on a case-by-case basis. More work is also needed to demonstrate the economic feasibility of these materials (including the regeneration process) for use in small-scale NSSS; field testing and pilot-scale studies will be critical for evaluating long-term viability of HAIX. Overall, there are ample opportunities for fundamental R&D in synthetic ion exchange resins, as well as modified and unmodified natural zeolites, for nutrient removal in NSSS^48,57.

Membrane-based separations

Membrane-based separation technologies are emerging as promising methods of nutrient removal for small-scale water treatment systems. Both active (energy required to drive separation; e.g. electrodialysis) and passive (separation driven by chemical equilibrium; e.g. osmosis, ion-selective membranes) membrane separation methods are dynamic areas of research for nutrient removal applications. For example, forward osmosis from source separated urine was recently shown to give at least 50% N and 93% P recovery into a diluted fertilizer draw solution. However, typical issues of membrane scaling and fouling were found to increase the operating costs of this system, due to the need for membrane replacement⁵⁸.

All membrane-based technologies suffer from a few key limitations in the membranes themselves, including membrane fouling and poor selectivity⁵⁹. However, important fundamental advances are continually being made in the development of tailored membrane materials⁶⁰. For example, highly selective membranes have recently been fabricated using covalent organic frameworks with tunable functional groups and nanoporous structures⁶⁰. However, cost and scalability of any new membrane materials must be considered from the earliest stages of development for application in NSSS, particularly for deployment in low- and middle-income countries. Opportunities also exist in modification of existing membranes and processes to tailor them for use in NSSS. Membrane surface modification can be used to increase hydrophilicity and membrane selectivity. Surface modification was recently used to demonstrate >89% ammonium removal in a non-optimized forward osmosis system⁶¹. Optimization of operating conditions and in depth techno-economic analyses of nutrient recovery may make membrane-separation-based technologies viable options under certain conditions. In particular, membrane fouling can be largely mitigated when membranes are used in “tertiary” nutrient removal processes (i.e. after COD removal).

Another promising method for mitigating membrane fouling is through electrodialysis reversal (EDR). In this process, the electrical polarity is switched periodically between the anode and cathode material to enable in situ self-cleaning of the electrode and membrane surfaces⁵⁹. EDR is already widely used in water desalination technologies, and one company (Saltworks Technologies Inc.) has developed a modular EDR cell stack demonstrating >95% ammonia removal in a municipal WWTP. Whether electrodialysis can be cost effective on a small scale and effectively handle more concentrated wastewater (blackwater) for use in NSSS will need to be demonstrated.