Maintaining the quality of water is a global challenge in this era of rapid industrialisation and urban growth, writes Yan Yang, RPS Group, and Xinmin Zhan, Ryan Institute/MaREI, NUI Galway.

In Ireland, a mere 52.8% of surface waters (rivers, lakes, transitional, and coastal waters) are of good or high ecological status, while the remaining 47.2% are classified as moderate, poor or bad. The overall decline rate is 4.4% in surface water quality since 2015 (EPA Ireland, 2019b).

Very challenging for Ireland

The decline in water quality makes it very challenging for Ireland to meet the water quality standards and objectives set out in the EU Water Framework Directive.

The decline of water quality in Ireland is mostly due to increased nitrogen (N) and phosphorus (P) runoffs from agricultural lands and urban wastewater sources. High frequency of flooding events has resulted in higher loads of N and P in downstream catchments.

Wastewater from 39 of the 169 large urban areas (up to 23%) requires more treatment to remove nutrients (N and/or P) to meet the specified standards (EPA Ireland, 2019a). Raw sewage from the equivalent of 77,000 people in 36 towns and villages is still released into the environment without any treatment (EPA Ireland, 2019a).

The elevated N and P concentrations in the water bodies increase the likelihood of excessive growth of algae and aquatic plants in surface waters, further deteriorating the water quality.

Challenges of nitrogen and phosphorus removal in urban WWTPs

In urban wastewater treatment plants (WWTPs), nutrient removal is generally achieved in the secondary treatment activated sludge process.

Nitrogen is removed from wastewater through oxidation of ammonium to nitrate (i.e. autotrophic nitrification), followed by reduction of nitrate to nitrogen gas (i.e. heterotrophic denitrification), and phosphorus is removed through enhanced biological phosphorus removal by heterotrophic phosphorus accumulating organisms and/or by dosing iron or aluminum chemicals.

These processes have high operational costs and high sludge production. In the secondary treated effluent, typical N and P concentrations are in the range of 10–15 mg N/L and 0.5–1.0 mg P/L, respectively.

EU Water Framework Directive regulates that the nutrient discharge limit is 10 mg/L for N and 1 mg/L for P in municipal WWTPs with population equivalent capacity of >100, 000.

More stringent discharge permits

Adopting more stringent discharge permits (e.g. TN<2.2 mg/L, TP<0.15 mg/L, which are currently in use by the Dutch water boards) as discharge guidelines for sensitive water bodies, is becoming a general trend in many countries.

Therefore, WWTPs may need tertiary treatment, like chemical polishing, sand filtration, membrane separation and post-denitrification with the addition of external organic carbon, to achieve stable performance and meet the stringent discharge limits.

The high energy and chemical inputs, complex operation of conventional secondary and tertiary wastewater treatment make it only a matter of time to switch to cost-effective, simple, and applicable technologies to achieve high effluent quality.

Iron sulfide as media in biofilter and wetland systems

Iron sulfide minerals (pyrite, chemical formula FeS₂; pyrrhotite, chemical formula Fe_1-xS, x = 0-0.125; mackinawite, chemical formula FeS) are abundant and widespread in mantle rocks and meteorites, notably in the US, Peru, Germany, Russia, Spain, South Africa, China and so on.

Irish people are quite familiar with pyrite because more than 20,000 properties have been contaminated with pyrite back-fill, which resulted in damage to the floors, walls and foundations. These iron sulfide minerals can be used as an excellent material for wastewater treatment.

Denitrification, removing nitrogen from wastewater, can be autotrophic or heterotrophic with the latter commonly used. The difference between the two is that in autotrophic denitrification, autotrophic denitrifiers reduce oxidised nitrogen (nitrate or nitrite) to nitrogen gas without using organic carbon as an energy source, while in heterotrophic denitrification, denitrifiers reduce nitrate or nitrite using organic carbon as an energy source.

Unique elemental component

Due to the unique elemental component of iron sulfides, which are made of reduced iron and sulfur, they can be utilised by sulfur oxidising bacteria, which are autotrophic denitrifiers, to reduce nitrate to nitrogen gas. The reaction can be expressed with the following equation:

Ferric iron and its oxyhydroxide products can remove phosphorus by precipitation and adsorption. Iron sulfides with a high durability can serve as the energy source and biofilm substratum to support bacterial growth. Autotrophic denitrification produces significantly less sludge compared to heterotrophic denitrification.

Iron sulfides can be used as media in biofilters and wetlands, and have been proved effective in simultaneous removal of N and P from wastewaters which have a low carbon/nitrogen ratio like secondary effluent of municipal WWTPs and agriculture runoff, at low cost and low energy input (Liang et al., 2019; Yang et al., 2017; Zhang et al., 2019).

Iron sulfides based biofilter can reduce N of 21.1 mg/L and P of 2.6 mg/L from secondary municipal effluent to as low as 1.89 mg/L and 0.34 mg/L, respectively (Li et al., 2016).

Three-year pilot study

A three-year pilot study using iron sulfides as the major media in constructed wetlands has demonstrated that average total N and total P removals were 69.4 % and 87.7 % with 4.0 mg N/L and 0.25 mg P/L in the treated water, respectively (Ge et al., 2018).

The release of trace metals from iron sulfides biofilters doesn’t show any concern to water quality, and no negative effects of iron sulfides on the growth of wetland plants are found.

To achieve optimal nutrient removal performance, the hydraulic retention time (HRT) of iron sulfides based constructed wetlands is 60-72 h, which is comparable to traditional constructed wetland systems with HRT of two to six a day.

The kinetics of iron sulfide biofilters can be improved by increasing the specific surface area of iron sulfides materials. This can be done by manufacturing natural iron sulfides into nanostructured materials.

Our lab-scale study has shown that at a hydraulic loading of 4.2 m³/m²∙d, average concentrations of 0.05 ± 0.01 mg N/L and 0.03 ± 0.01 mg P/L < 1 mg/L for both N and P were achieved when using nanostructured iron sulfides as the media in the biofilters to treat real secondary effluent (Yang et al., 2017).

Potential of technology for tertiary wastewater treatment

The low concentrations of N and P in the treated water using the nanostructured iron sulfide biofilters indicate the potential of this technology for tertiary wastewater treatment and in meeting strict discharge standards.

The total iron (Fe) concentration in the treated water is below 2.5 mg/L (Yang et al., 2017). However, even iron concentration as little as 0.3 mg/L can cause water to turn a reddish brown colour.

The concentration of Fe is not regulated in the EU Regulations for Urban Waste Water Treatment, and is only used as indicator of < 200 µg/L in the drinking water regulated by EU drinking water directive.

Therefore, the low level of release of Fe does not impact on public health and safety as Fe concentrations of 1–3 mg/L is acceptable for people drinking anaerobic well-water (World Health Organization (WHO, 2003).

While, addition of lime into the treated water would help to reduce Fe and the colour, if necessary.

Summary

Iron sulfides based biofiltration technology can be used as an effective tertiary wastewater treatment for attaining stringent N and P emission limits.

While, because all completed studies were conducted in laboratory and small pilot-scale testing systems, a large-scale demonstration testing should be built and tested in Ireland to address this novel technology as tertiary treatment for N and P removal.

Authors: Yan Yang, RPS Group and Xinmin Zhan, civil engineering/Ryan Institute/MaREI, National University of Ireland, Galway

References

1.) EPA Ireland. 2019a. Urban Waste Water Treatment in 2018.

2.) EPA Ireland. 2019b. Water Quality in Ireland 2013-2018.

3.) Ge, Z., Wei, D., Zhang, J., Hu, J., Liu, Z., Li, R. 2018. Natural pyrite to enhance simultaneous long-term nitrogen and phosphorus removal in constructed wetland: Three years of pilot study. Water Res, 148, 153-161.

4.) Li, R., Morrison, L., Collins, G., Li, A., Zhan, X. 2016. Simultaneous nitrate and phosphate removal from wastewater lacking organic matter through microbial oxidation of pyrrhotite coupled to nitrate reduction. Water Research, 96, 32-41.

5.) Liang, Y., Wei, D., Hu, J., Zhang, J., Liu, Z., Li, A., Li, R. 2019. Glyphosate and nutrients removal from simulated agricultural runoff in a pilot pyrrhotite constructed wetland. Water Research, 115154.

6.) World Health Organization (WHO). 2003. Iron in drinking water: background Document for Development of WHO Guidelines for Drinking Water Quality

7.) Yang, Y., Chen, T., Morrison, L., Gerrity, S., Collins, G., Porca, E., Li, R., Zhan, X. 2017. Nanostructured pyrrhotite supports autotrophic denitrification for simultaneous nitrogen and phosphorus removal from secondary effluents. Chemical Engineering Journal, 328, 511-518.

8.) Zhang, Y., Wei, D., Morrison, L., Ge, Z., Zhan, X., Li, R. 2019. Nutrient removal through pyrrhotite autotrophic denitrification: Implications for eutrophication control. Science of the Total Environment, 662, 287-296.