Reuse, Recovery & Stormwater
Nutrient Recovery Companies
Phosphorus and nitrogen recovery, struvite, ammonia stripping, and bioreactor-based nutrient reuse.
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Nutrient Recovery from Wastewater: Struvite Precipitation, Ammonia Stripping, and Algae-Based Systems
Nutrient recovery from wastewater and sludge streams recovers phosphorus and nitrogen as marketable products, aligning with circular economy principles and reducing energy-intensive synthetic fertiliser production. Phosphorus recovery by struvite (MgNH4PO4.6H2O) precipitation: in anaerobic digester centrate and belt press filtrate (high NH4+, PO4 3-, Mg2+ concentrations), raising pH to 8.5 to 9.0 by CO2 stripping or NaOH dosing while adding MgCl2 or MgO (Mg:P molar ratio 1.1 to 1.3) precipitates struvite at greater than 80 percent P recovery. Commercial systems (Ostara Pearl, Multiform Harvest, Eliquo PHOSPAQ) produce struvite prills (1 to 5 mm granules) with typical analysis 5.7 percent N, 12.5 percent P2O5, 9.9 percent MgO - equivalent to a slow-release fertiliser.
Ammonia recovery from digestate and sludge liquors uses: (1) Ammonia stripping - raising pH above 10 with NaOH or lime, passing air through packed tower to strip NH3 gas, then absorbing in sulphuric acid to produce ammonium sulphate solution (AS, 24 to 40 percent concentration, sold as nitrogen fertiliser at market price $100 to $300 per tonne N); (2) Membrane contactors (hollow fibre gas-permeable membranes, lower energy than packed towers but higher capital cost); (3) Electrochemical stripping (applying electrical potential to drive NH3 through selective membrane). Heat-assisted ammonia stripping at 50 to 70 degrees C using waste heat from CHP reduces air flow requirements by 50 to 70 percent versus ambient stripping. Struvite precipitation controls P and NH4 simultaneously; stripping addresses the residual ammonia.
Algae-based nutrient recovery uses photobioreactors or open raceways to cultivate microalgae (Chlorella, Spirulina, Scenedesmus) on wastewater centrate or secondary effluent, assimilating N and P into biomass. Algae biomass (50 percent protein, 10 to 30 percent lipid, 10 to 20 percent carbohydrate) is harvested for high-value applications (aquaculture feed, nutraceuticals) or energy recovery (lipid extraction for biodiesel, biomass gasification). P removal from secondary effluent to below 1 mg per L and N to below 10 mg per L is achievable in warm climates with adequate light. Challenges: seasonal light limitation in temperate climates, high harvesting cost (centrifugation at $0.50 to $2.00 per m3 of culture), and regulatory restrictions on food-chain use of biomass grown on sewage. Pilot and commercial systems operational in Europe, Australia, and US.
Frequently Asked Questions
What is struvite and why is it recovered from wastewater?
Struvite (magnesium ammonium phosphate hexahydrate, MgNH4PO4.6H2O) is a crystalline mineral that spontaneously precipitates in anaerobic digestion systems when magnesium, ammonium, and phosphate ions reach supersaturation. It forms as a hard scale in pipes, pumps, and centrifuges, causing significant maintenance problems. Controlled struvite precipitation in dedicated reactors recovers phosphorus and ammonium as a slow-release fertiliser prills (granules), achieving two benefits: (1) removal of problem struvite scale in plant pipework and equipment; (2) recovery of a marketable fertiliser product containing 5.7 percent N, 12.5 percent P2O5, 9.9 percent Mg. Commercial value: $200 to $500 per tonne of struvite, sold as a specialist controlled-release fertiliser brand (e.g. Crystal Green by Ostara). Struvite recovery from a 500,000 population WWTP can produce 100 to 300 tonnes per year, offsetting some operational costs.
How much phosphorus can be recovered from wastewater?
Phosphorus recovery potential from wastewater treatment: (1) Struvite precipitation from sludge liquors (anaerobic digestion centrate or sludge thickening overflow) achieves 80 to 90 percent P recovery from the liquor stream, which represents 15 to 25 percent of total plant influent P load; this produces 5 to 10 percent of total plant P as struvite product; (2) Enhanced biological phosphorus removal (EBPR) plus P release into sidestream and struvite precipitation: total P recovery of 30 to 40 percent of influent load achievable; (3) Thermochemical treatment of sewage sludge ash (at 450 degrees C with Na2CO3): converts calcium phosphate in ash to water-soluble Na-P compounds; P recovery 80 to 90 percent of sludge ash P; incineration of all UK sewage sludge would yield 26,000 to 30,000 tonnes P per year (vs 100,000 tonnes P consumed in UK fertiliser annually). Phosphorus is a finite, non-substitutable resource for agriculture; European deposits are limited and geopolitical concentration (70 percent in Morocco) makes domestic recovery strategically important.
Can ammonia be recovered from wastewater for use as fertiliser?
Yes. Ammonia recovery from wastewater sidestreams produces ammonium sulphate (AS) solution at 24 to 40 percent concentration, which is a direct nitrogen fertiliser (typical price $100 to $300 per tonne N, equivalent to $50 to $135 per tonne of 24 percent AS). The process: wastewater centrate or concentrate (NH4-N typically 500 to 1,500 mg per L) is alkalified to pH above 10 by NaOH addition, converting NH4+ to volatile NH3; ammonia is stripped by air in a packed column; the NH3-laden air is absorbed in a counter-current sulphuric acid solution to form ammonium sulphate. Typical N recovery: 80 to 95 percent of influent NH4. Challenges: high NaOH cost (0.1 to 0.2 kg NaOH per g NH4-N removed), energy for air blowing, and H2SO4 cost. Waste heat from CHP or industrial processes at 50 to 70 degrees C reduces air and NaOH requirements significantly. Market demand for AS is well-established in agriculture; proximity to arable farming areas improves economics.
What regulations apply to recovered nutrients?
Regulatory status of recovered nutrients depends on the recovery route and end use. Struvite from sewage: in the UK, classified as an end-of-waste product (Environment Agency 'Quality Protocol for struvite') if it meets quality standards for heavy metals and organic contaminants, enabling sale as a fertiliser without waste carrier obligations. EU Fertilising Products Regulation (2019/1009): creates an EU-wide market for recovered struvite and other secondary raw materials as CE-marked fertilisers subject to limits for Cd (60 mg per kg P2O5), Cr(VI), and other metals. US: state-specific regulations apply; most states allow land application under NPDES permit conditions or under state fertiliser registration. Sewage sludge (biosolids) for land application: US EPA 40 CFR Part 503 (Class B or Class A depending on treatment); UK Sludge Use in Agriculture Regulations; EU is reviewing the Sewage Sludge Directive with stricter standards anticipated by 2025 to 2027 to address organic micropollutants (PFAS, pharmaceuticals).
A 350,000 PE WWTP with thermal hydrolysis pretreatment (THP) and mesophilic anaerobic digestion was generating 12,000 tonnes dry solids per year. Struvite scaling was blocking centrifuge feed pipes and causing 40 to 60 hours of unplanned downtime per year. The utility wanted to address the scaling problem while recovering value from the digester centrate.
An Ostara Pearl reactor system was installed to treat the centrifuge reject stream (800 m3 per day, 180 mg per L P, 900 mg per L NH4-N). MgCl2 dosing at a Mg:P molar ratio of 1.2 and pH control to 8.7 precipitated struvite granules (3 to 4 mm diameter) at 85 percent P recovery from the centrate. The recovered struvite (branded Crystal Green) was bagged and sold to a specialist horticultural distributor.
Struvite scaling incidents in centrifuge pipework reduced by 90 percent, recovering 45 hours per year of productive operating time. Annual struvite production was 95 tonnes per year, generating 28,500 GBP per year revenue at 300 GBP per tonne. Phosphorus returned to agricultural land via the product reduced the WWTP's land application sludge P load by 15 percent.
Questions to Ask Shortlisted Providers
- 1
What is the phosphorus and ammonia concentration in the target sidestream (centrate or filtrate)?
Struvite precipitation efficiency depends on P and NH4 concentrations; dilute streams (below 50 mg per L P) are rarely economical to treat without concentration.
- 2
Has the struvite end-of-waste Quality Protocol been reviewed for the specific sludge source and product quality?
The EA Quality Protocol sets metal limits and certification requirements; sludge from industrial catchments may contain metals that prevent product from meeting end-of-waste status.
- 3
What is the ammonia stripping technology proposed and what is the carbon source cost for biological denitrification of the stripped liquor?
Ammonia recovery requires alkali (NaOH or lime) and acid (H2SO4); chemical cost is the dominant OPEX driver and must be quantified against the ammonium sulphate revenue.
- 4
Has a feasibility study been completed for the full nutrient mass balance, including impacts on the liquid stream consent?
Returning nutrient-rich sidestreams to the main flow can compromise the final effluent consent if the WWTP is near its nitrogen or phosphorus capacity.
- 5
What granule size and purity specification does the intended market (agricultural or horticultural) require?
Struvite market price and offtake agreement terms depend on product specification; smaller granules and lower purity fetch lower prices or may not find an offtaker.
What Drives Cost in This Category
Struvite reactor systems for 50 to 200 m3 per day centrate cost 500,000 to 2,000,000 GBP installed; civil works for a new building or reactor chamber can double project cost on constrained WWTP sites.
MgCl2 represents 40 to 60 percent of struvite OPEX; MgO or recovered magnesium can reduce cost but add process complexity.
Without a confirmed offtake buyer and transport logistics for the product, recovered nutrients may require landfill disposal, converting a revenue stream into a cost.
Nutrient-rich sidestreams returned to the main flow can increase N and P loading by 15 to 25 percent; if the works is near its consent limit, capacity expansion may be needed alongside the recovery system.
Key Regulations & Standards
Sets the end-of-waste criteria for struvite recovered from sewage sludge in England and Wales; product must meet heavy metal limits for Cd, Cr, Ni, Pb, Hg, and Cu to be classified as a non-waste fertiliser.
Creates a CE-marked fertiliser category for struvite recovered from wastewater; enables cross-border sale as a recognised fertiliser product subject to conformity assessment and labelling requirements.
Governs land application of sewage biosolids as a complementary route for phosphorus and nitrogen recycling; metal limits and application rates must be adhered to for all amended land.
Nutrient recovery installations at WWTPs require an Environmental Permit variation or standalone permit covering process emissions, waste management, and product quality standards.