How might PUB recover phosphorus from dewatered sludge, dewatering centrate or sewage sludge incineration ash (SSIA) cost-effectively for safe and sustainable local vegetable production via hydroponics cultivation?

• Water Reclamation (Plants) Department – Planning, Development & Corporate Services Division

• Technology Department – Technology Development Division

Phosphorus is a critical yet non-renewable resource for modern agriculture. Its current global reserves are ex​pected to be depleted in 50 - 100 years. The phosphorus scarcity issue poses a serious threat to food security globally. Under the ’30 by 30’ goal, Singapore aspires to build up the capability and capacity of our agri-food industry to produce 30% of our nutritional needs sustainably by 2030. With limited land, Singapore has identified hydroponics as one Agri-tech that can help speed up the growing cycle of our homegrown tech without compromising on nutritional value.

Due to the potential depletion of phosphorus as well as more stringent regulations on the discharge quality to water courses, there is a growing interest worldwide to recover phosphorus from municipal wastewater treatment plants (WWTPs)¹. PUB hence calls for a solution that can recover phosphorus from the following waste streams from our local water reclamation plants (WRPs) that are suitable for local agricultural use:

PUB had conducted several trials with SFA to investigate the applicability of phosphorus recovery from waste streams from WRP.  The known limitations are as outlined below:

• Struvite that can be recovered from dewatering centrate and dewatered sludge was assessed to be very slow releasing in solution, and cannot be readily dissolved in nutrient solution for hydroponics cultivation; and
• Application of dewatering centrate (blended with commercial nutrient solution at different mixing ratios) was not feasible due to bioaccumulation of contaminants in the harvested hydroponically grown vegetables. Growth yield was also found to be impeded.

Footnote
¹For instance, Germany stipulates phosphorus recovery from WWTPs with at least 100,000 population equivalents (PE) mandatory by 2029.

PUB is interested in seeking partners to work with who can identify phosphorus recovery process that can be integrated with PUB’s used water treatment processes or paired with incineration plants to recover phosphorous from the various waste streams, such as dewatered sludge, dewatered centrate and SSIA.

The phosphorus product recovered must be of quality, free of contaminants and readily soluble in water so that it can be applied as part of nutrient solution for hydroponics cultivation. The overall cost-benefit analysis of the solution, and the consistency of the phosphorus produced across batches for hydroponics cultivation shall be demonstrated.

Considerations to be addressed include:

• The recovered phosphorus product should be readily soluble in water so that it could be applied as part of nutrient solutions for hydroponics cultivation.
• The recovered phosphorus product shall be free of contaminants such as pharmaceutical compounds, heavy metals or other inhibitory chemicals.
  
• Hydroponics vegetable field trials shall be carried out on the commonly consumed and grown crops (e.g. Chinese Cabbage, Xiao Bai Cai) using the recovered phosphorus product, against a commercial hydroponic nutrient solution to understand the extent of solubility and plant uptake. Bioaccumulation of contaminants shall not be present within the crop batches.
• The developed phosphorus recovery process should be compact and easily integrated with the current used water treatment processes within PUB or paired with incineration plants. It should also be cost-effective when benchmarked against the value of commercial fertilizer solutions. The cost-benefit analysis shall be clearly articulated in the proposed solutions.
• Consistency of the phosphorus produced across different batches shall be demonstrated.
• The use of the recovered phosphorus product in agricultural applications should keep in mind agencies’ regulations.

There are commercial solutions that can readily recover phosphorus from municipal wastewater centrate, found typically in the form of struvite (NH₄MgPO₄·6H₂O). However, precipitated struvite was not found to dissolve easily in solution, making it difficult for use in hydroponics applications. 

There are also some companies exploring the potential for recovery of phosphorus from sewage sludge incinerated ash (SSIA).

• PUB's Used Water Treatment Process

• SFA's 30 by 30 Vision
  

After Selection

• Samples of the various waste streams for the proposed demonstration study.

Total project period – less than 18 months

A prototype or equivalent that can effectively recover phosphorus from the waste stream(s) of PUB’s WRP in a form suitable for local agricultural use. Techno-economic analysis should also be included to illustrate the merits of the proposed solution.

If the preliminary study or proof-of-concept is successful, PUB may further scale up the solution.

Q: What is the optimum pH range for the hydroponic fertiliser solutions?

A: The optimum pH range is dependent on several factors, such as the nature of the crops grown. The typical pH range used by farmers is around ~6.5.

Q: Based on the presentation, impeded growth of crops was observed in the trial involving the use of dewatering centrate mixed with commercial nutrient solution. What types of vegetables have been tested? Does the degree of impedance vary among different types of vegetables?

A: PUB/SFA has previously conducted crop tests on Xiao Bai Cai (Chinese Cabbage) and Lettuce, which are some of the most commonly consumed vegetables in Singapore.  PUB has not studied the degree of impedance among different crop types. However, we observed noticeable differences in the crops grown using dewatering centrate mixed with commercial nutrient solution and the crops that are grown purely using commercial nutrient solution.

Q: What is the typical pH of the dewatering centrate?

The pH of the dewatering centrate based on six months of routine sampling is approx. 7.7.

Q: Is PUB currently producing struvite? If yes, could struvite be used as a source of phosphorus to develop a soluble fertiliser for hydroponics purpose?

A: PUB's Used Water Treatment Operations currently does not recover struvite as a main product. Previously, PUB had tried crystallising struvite from the dewatering centrate stream for hydroponics applications, but observed that it had limited solubility in hydroponic solutions. We will consider any cost-effective solutions that can dissolve struvite in the hydroponics solutions without affecting the optimal pH required for farming operations. The costs of producing struvite and dissolving struvite (if any) will need to be accounted for in the techno-economic analysis.

Q: Are there any requirements for the solution to be set up in an indoor or outdoor environment?

A: Both outdoor and indoor setups are acceptable. Solutions that can be set up in an indoor environment will be more aligned with future plans to advance towards vertical or multi-tier farming systems.

Q: What is the scale of the pilot required for this challenge?

A: We recognise that there are several scopes of work that could be covered in the proposed project. This could include the development of the phosphorus recovery process and the crop tests that needs to be performed in order to validate the use of the resulting product as a fertiliser for hydroponics purposes.

Applicants will need to propose the scope of the pilot, considering the budget limitations due to the pilot funding that is capped at S$250,000.

Q: Is PUB open to looking at phosphorus recovery from upstream treatment processes instead of the listed waste streams?

A: The concentration of phosphorus in the upstream processes are typically too low for the return-on-investment (ROI) to be justified. While PUB is open to explore new technologies to recover phosphorus, the ROI should be justified.

Q: Is PUB open to solutions that involves the addition of chemicals in the upstream process?

A: In Singapore's context, biological-based wastewater treatment methods is largely preferred to conventional chemical precipitation methods for minimising OPEX associated with sludge disposal and chemical costs.

Q: For the techno-economic analysis for the phosphorus recovered, what is the quantity considered?

A: Quantity estimates of the available phosphorus for the different waste streams can be found in the table listed under the Background and Current Practice section. We suggest referencing the figures provided in the table for your techno-economic analysis.