Introduction
Large amounts of mineral nutrients are being released into the environment through various waste streams, leading to pollution of soil, water, and air. However, increasingly stringent legislation is driving the reduction of wastewater pollutants, particularly nitrogen (N) and phosphorus (P), while also promoting circular economy initiatives. At the same time, resource depletion and growing global demand for mineral nutrients have led to price pressures and a search for alternative nutrient sources, such as industrial wastewaters. Recovering nutrients from wastewater offers dual benefits: it removes impurities from water, making it reusable, and concentrates these impurities into forms that can be used as recycled nutrients.
Industrial wastewaters pose challenges for nutrient recovery due to their typically low nutrient concentrations, complex compositions, and the presence of contaminants such as heavy metals and organic pollutants. Low nutrient concentrations reduce the economic feasibility of recovery, while the complexity of wastewater makes selective extraction of desired nutrients difficult. Contaminants may accumulate in recycled nutrient products, potentially rendering them unsuitable for agricultural use. Furthermore, strict regulations govern the use of recycled materials in agriculture. Addressing these challenges often increases the cost and complexity of nutrient recycling processes.
Nutrient-Rich Industrial Wastewaters
Unlike municipal wastewater, industrial effluents vary significantly depending on the industry type and production cycles. Seasonal fluctuations in wastewater quality and quantity further complicate the design of standardized nutrient recovery processes. Findings from the TYPKI project, funded by Business Finland, VTT, the University of Oulu, and industry partners, revealed that industrial wastewater typically contains low nutrient concentrations, which are nonetheless too high to be discharged into the environment without treatment. Nutrient recovery was found to be most feasible when integrated into water purification processes. Since all generated streams must be managed during purification, utilizing concentrates for nutrient recovery becomes advantageous.
Mine waters generally contain low levels of organic nutrients, but some mineral nutrients can still be recovered. For example, wastewater from chemical leaching of concentrates can contain ammonium salts, albeit at low concentrations, necessitating additional concentration and purification steps. Tailings pond waters can be rich in sodium sulphate (Na₂SO₄) and can also contain potassium (K) and magnesium (Mg). Open pit water from mines may contain residual explosives, such as ammonium nitrate (NH₄NO₃). Although the concentrations are low, they often still are too high for direct discharge, making recovery from water purification concentrates a viable option. While phosphorus is typically not a major concern in mining, its use in metal refining and metal processing applications such as metal surface treatment, particularly involving phosphoric acid, can lead to elevated phosphorus levels in wastewater.
In the pulp and paper industry, N and P are present in wastewater, but often not in sufficient quantities to support biological treatment without supplementation. However, scrubber water from flue gas cleaning can contain valuable nitrogen compounds, especially nitrite (NO₂⁻) and nitrate (NO₃⁻) salts, which offer significant potential for nutrient recovery at the source.
The food industry generates some of the most nutrient-rich wastewaters due to the presence of organic nutrients and biological residues. In the European Commission funded Afterlife project, VTT studied wastewaters from dairy, beverage, and confectionery factories for the recovery of valuable compounds. These wastewaters, rich in organic content, were found suitable for microbial processes and nutrient recovery for fertilizer production. Dairy and meat processing wastewaters contain proteins, fats, and phosphorus, while beverage and confectionery wastewaters are rich in sugars, polyphenols, and nitrogenous compounds.
Biogas production processes, often integrated into industrial wastewater treatment, generate a liquid by-product known as digestate. This digestate is rich in ammonium, phosphorus, potassium, and organic carbon. It can be used directly as fertilizer or further processed to recover nutrients, such as struvite, through precipitation.
Technologies for Nutrient Recovery
The choice of nutrient recovery technology depends on the quality and quantity of the wastewater, as well as the desired quality of the final nutrient product. While the goal is to develop simple recovery concepts, these often involve multiple technologies in sequence. Since no universal solution exists for nutrient recovery from industrial wastewater, two examples from the TYPKI project are presented below.
In one case, nanofiltration (NF) and reverse osmosis (RO) were used to treat concentrate leaching wastewater, producing reusable water as permeate with low concentrations of impurities. A calcium (Ca) concentration below 30 mg/L is preferred to achieve high water recovery in NF and RO processes. This can be achieved through precipitation using oxalic acid or sodium carbonate. The purified water can constitute up to 95% of the effluent volume, which results in a 20-fold reduction in the volume of nitrogen-containing water, effectively concentrating the nutrients. For nitrogen recovery, a membrane contactor (MC) can be used to extract ammonia from the concentrate, producing a pure 30% ammonium sulphate ((NH₄)₂SO₄) solution.
In another example, nutrients from open pit water were recovered using NF as a pretreatment step before RO concentration. The optimal NF membrane exhibited high sulphate rejection and low nitrate rejection. This process increased the nitrate concentration in pit water from 110 mg/L to 3,900 mg/L. However, further concentration and purification of nitrate compounds, such as potassium nitrate (KNO₃), require additional technologies, such as crystallization. The additional step increases production costs. The estimated operating cost for NF and RO is as low as 0.5 €/m³ (assuming 0.1 €/kWh), whereas cooling crystallization costs exceed 4 €/kg of KNO₃.
Market and Products
The market and application of recycled nutrients are governed by a combination of EU-wide and national regulations. Strict requirements for purity and concentration, driven by safety, performance, and market acceptance, make nutrient recycling a challenging business. Moreover, the presence of well-established nutrient sources and end-users further raises the entry barrier.
However, upcoming policy developments are expected to promote circular economy initiatives. Wastewater treatment plants may be required to recover a portion of nutrients, while chemical companies could be encouraged, or even mandated, to incorporate recycled nutrients into their products.
Current separation technologies already enable the simultaneous production of purified water and the recovery of concentrated, high-purity nutrient products. The key question remains: when will this become broadly profitable?
Hanna Kyllönen
Senior Scientist
VTT Technical Research Centre of Finland Ltd
Finland
hanna.kyllonen@vtt.fi
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