Smelly Self-Sufficiency?

About two weeks ago, we paid a visit to our campus’ own BCA Green Mark (Platinum) certified (BCA, 2018) net-zero energy building at SDE4. While SDE4’s energy-saving, energy-producing, and innovative construction measures were intriguing, what got my water-obsessed brain fired up were its water recycling facilities.

Under BCA’s Green Mark Criteria (BCA, 2016), points are awarded where non-potable water scavenged is used to reduce potable water usage. SDE4 achieves this using a rainwater harvesting system, where two-thirds is used for flushing and irrigation of gardens, and the remaining is fed through a simulated wetland called a bio-retention basin before being discharged into sewers, as seen below.

Schematic of SDE4’s rainwater harvesting and recycling system. Source:

Diagram of SDE4’s Bio-retention Basin. Source:

The Bio-retention Basin has a similar design to a Vertical Flow Wetland that is used for sewage treatment. Its abilities include reducing high BOD, suspended solids and pathogenic loads, and converting ammonia to nitrogenous oxides by wetland microbes. (Tilley, Lüthi, Morel, Zurbrügg, & Schertenleib, 2008). Source:

This essentially makes SDE4 one of our 4 national taps: a mini catchment area!

While walking around the learning trails, something under the sink outside the fifth-floor design studios caught my eye.

A small biological treatment tank I found outside the Design Studio.

Alternative baffles can be seen, giving wastewater pollutants a higher retention time inside the reactor for the bacteria do decompose the pollutants.

Since that was the first bioreactor I have seen in person, I couldn’t help but open it.

There are visible improvements in water clarity and amount of scum produced as the water proceeds through the bio-reactor.

As vile as it looks, the smell was not particularly objectionable and similar to a mangrove swamp, hinting its modus operandi.

From what I can surmise, the reactor design incorporates elements from an anaerobic baffled bioreactor as pictured below.

Exposure to aerobic conditions above the surface and anoxic conditions below the surface might increase the diversity of the microbial community in the reactor, thereby expanding the range of pollutants it can handle. This might be appropriate since Design Studio personnel handle resins and aerosol solvents that are prohibited under the SDA effluent discharge limits. Unfortunately, I couldn’t get clarification from the SDE on this instalment.

With the building’s energy budget as constrained as it is, this bioreactor fits right in since the bacteria do all the work without any extra energy (Tilley et al., 2008)! However, without any sort of disinfection system or a clarification tank to remove any remaining pathogens and microbes, recycling the resulting effluent might not be so wise (Tilley et al., 2008).

Hence, I decided to look for low-energy alternatives for water recycling solutions; and what I found was a two-for-one deal!

Several studies have investigated the applicability of incinerator fly ash and bottom ash as an adsorbent for wastewater pollutants. Conventionally, this is done using activated carbon, which is costly (Ahmaruzzaman & Gupta, 2011). Conversely, fly and bottom ash are produced by the truckload every day. They have proven themselves against a wide variety of simulated wastewaters including pharmaceuticals & petrochemicals (Sunil J. Kulkarni, Sonali R. Dhokpande, 2013) and dyes (Gupta et al., 2005) which may be produced by the Design Studio personnel. The resulting effluent could contribute to the non-potable water demands of the building.

These techniques have not been rigorously tested and applied industrially, unlike energy-intensive reverse osmosis or activated carbon (Ahmaruzzaman & Gupta, 2011). However, SDE4 provides an amazing platform for pilot scale fixtures to be tested such that they may be feasibly applied to other buildings to spur low-energy water recycling and self-sufficiency!

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Ahmaruzzaman, M., & Gupta, V. K. (2011). Rice husk and its ash as low-cost adsorbents in water and wastewater treatment. Industrial and Engineering Chemistry Research.

BCA. (2016). GREEN MARK FOR NON-RESIDENTIAL BUILDINGS NRB: 2015 including Hawker Centres, Healthcare Facilities, Laboratory Buildings and Schools. Building and Construction Authority Green Mark, 73. Retrieved from

BCA. (2018). BCA Awards 2018. Building and Construction Authority Green Mark, 85. Retrieved from

Gupta, V. K., Ali, I., Saini, V. K., Van Gerven, T., Van Bruggen, B. Der, & Vandecasteele, C. (2005). Removal of dyes from wastewater using bottom ash. Industrial and Engineering Chemistry Research.

Sunil J. Kulkarni, Sonali R. Dhokpande, J. P. K. (2013). Studies On Flyash As An Adsorbent For Removal Of Various Pollutants From Wastewater. International Journal of Engineering Research & Technology, 2(5), 1190–1195. Retrieved from

Tilley, E., Lüthi, C., Morel, A., Zurbrügg, C., & Schertenleib, R. (2008). Compendium of Sanitation Systems and Technologies. In Development. Retrieved from

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Phosphorus: Our Other Black Gold

It is quite literally one of the few elements that hold us together. Without it, almost every oxygen-respiring organism would slump into an organic soup (Ruttenberg, 2013) More importantly, it supplies energy to most aerobic organisms’ cells, allowing lifeless organic compounds to move, grow and well, be alive (Ruttenberg, 2013)!

The effects of phosphorus fertiliser use on crops. Source: Franklin D. Roosevelt Presidential Library and Museum (53227(1828)

In agriculture, phosphorus is a vital ingredient in fertilisers that prevent topsoil degradation after harvests have removed soil nutrients (Cordell, Drangert, & White, 2009). This phosphorus is mined from rocks (aptly named apatite) that have substantial concentrations of phosphorus (Daneshgar, Callegari, Capodaglio, & Vaccari, 2018), supplied by tectonic uplift of buried oceanic sediments as seen below (Ruttenberg, 2013).

The global phosphorus cycle. Upward arrows indicate phosphorus replenishment. Source: Ruttenberg et al. (2013)

Our society effectively relies on a resource that takes tens of thousands of years to replenish to feed an ever-growing population (Cordell et al., 2009; Ruttenberg, 2013)! Unsurprisingly, we exploit this finite resource like there’s no tomorrow (Daneshgar et al., 2018). We pump more phosphorus than the ground can hold, causing the excess to leach into our rivers and oceans; stimulating toxic algae blooms that kill fish and create anoxic dead zones when the nutrients have been exhausted (MIT, 2016).

The depletion of phosphorus reserves is a hotly debated topic (Daneshgar et al., 2018). Daneshgar et al. (2018) gave a tentative estimate of a few centuries, while Cordell et al. (2009) painted a grim few decades remaining of unequal phosphorus distribution between the rich and poor. As an environmentalist, the concept of relying on a finite resource gives me the same anxiety as a looming deadline; it’s an itch that I cannot tolerate.

Anthropogenic phosphorus budget, landfill and sewage losses are fairly significant. Source: Cordell et al. (2009)

Ultimately, the ideal outcome is closing the anthropogenic phosphorus loop and preserving natural phosphorus flows. The recovery of phosphorus from wastewater treatment, while not a large proportion of anthropogenic phosphorus loss, is still a significant point-source emitter that we can feasibly amend (Cordell et al., 2009; Daneshgar et al., 2018). Furthermore, the inherent global distribution of wastewater treatment plants (WWTP) could equalise the current oligopoly of phosphorus source countries (Daneshgar et al., 2018). At least, it’s better than twiddling our thumbs.

Source: PUB

In conventional WWTPs, most of the phosphorus in wastewater is removed in sludge to meet effluent discharge standards and lost when the sludge is incinerated and landfilled (Cornel & Schaum, 2009).

Phosphorus-recovering WWTPs typically utilise magnesium and calcium salts to crystallise phosphate fertilisers from wastewater, wet sludge or incinerated sludge ash after pre-treatment to remove other pollutants like heavy metals (Cornel & Schaum, 2009). Other novel approaches such as the use of algae sequestration have been tested, but trade away phosphorus recovery performance for the production of ready-made livestock feed and organic fertiliser (Shilton, Powell, & Guieysse, 2012).

In the pursuit of recovering and reusing a finite resource, we should not lose sight of other environmental objectives too. After all, it would be pointless if phosphorus recovery uses more energy than phosphorus extraction from apatite (Daneshgar et al., 2018) since we would be exchanging different forms of environmental degradation.

Just imagine if our upcoming Integrated Waste Management Facility had a zero-energy phosphorus recovery system; we could be one step closer to sustainable food self-sufficiency!

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Cordell, D., Drangert, J. O., & White, S. (2009). The story of phosphorus: Global food security and food for thought. Global Environmental Change.

Cornel, P., & Schaum, C. (2009). Phosphorus recovery from wastewater: Needs, technologies and costs. Water Science and Technology.

Daneshgar, S., Callegari, A., Capodaglio, A. G., & Vaccari, D. (2018). The potential phosphorus crisis: Resource conservation and possible escape technologies: A review. Resources.

Massachusetts Institute of Technology(MIT) (2016). Eliminating depletion and environmental damage with efficient phosphorus use and reuse. Mission 2016. Retrieved from

Ruttenberg, K. C. (2013). The Global Phosphorus Cycle. In Treatise on Geochemistry: Second Edition.

Shilton, A. N., Powell, N., & Guieysse, B. (2012). Plant based phosphorus recovery from wastewater via algae and macrophytes. Current Opinion in Biotechnology.

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End of the Line

So, you’ve finished your apple and are left with its core, and you do not have access to a composter; what would you do?

Throw it in the bin, I hope! From there, our trash ends up at one of four Waste-to-Energy incinerators, where the majority of its water content and bulk is reduced to bottom ash (BA) to save space. This BA is then transferred together with sludge ash from PUB’s wastewater treatment plants (WWTP) (PUB, 2016) and Toxic Industrial Waste Collectors* by barge to Pulau Semakau, where it is dumped into the sea.

Bottom ash is transferred by barge to this transfer station

But wait, would that mean that hazardous substances from toxic industrial ash could perhaps leach into the sea? This could very well be the case. BA usually contains heavy metals(Lin, Wu, & Liu, 2007) and refractory halide-containing organic compounds, possibly from BOCs in WWTP sludge (Dung, Vassilieva, Swennen, & Cappuyns, 2018). Needless to say, these pollutants really should not be leaching into the sea due to their mutagenic or biocidal effects(Manaia et al., 2018; Tchounwou, Yedjou, Patlolla, & Sutton, 2012). Their persistence to biodegradation causes them to bio-accumulate and bio-magnify in apex predators and eventually us.

With these concerns in mind, I had the privilege of going to Pulau Semakau twice to find out how Singapore deals with the BA leachate issue. What I found clarified my woes, but raised other questions too.

Bottom ash is carried by these dump trucks to the floating bottom ash dumping platform

Now at Phase II of development, BA is being dumped into a large enclosed lagoon as seen below.

Map of Semakau landfill. Full credit goes to NEA for the map.

This lagoon is enclosed by granite bungs with a layer of impermeable material extending all the way to the sea bed, seen here.

Granite bung with an impermeable membrane at Phase II lagoon

As more ash is filled in, the lagoon’s water level rises, necessitating overflow discharge into the sea. This discharge is treated by a floating WWTP such that it meets the allowable limits for trade effluent discharge into a watercourse. The floating WWTP can be seen in the distance below.

Pulau Semakau’s floating WWTP in the distance

Mockup of Phase II lagoon showing the floating dumping platform (longer) and the floating WWTP (shorter). Full credit goes to NEA for the mockup.

I also found an intriguing infographic on the technical specifications of the WWTP:

Infographic on the Semakau WWTP. Full credit to Memiontec Pte Ltd. and NEA for the infographic

We can conclude from these observations that direct exposure of BA leachate to seawater is entirely prevented.

However, a particular remark by one of our guides struck me: that at the bottom of the lagoon is the original seabed. This would mean that years after Phase II has been filled, the BA could become highly acidic after being repeatedly exposed to haze, polluted air from nearby Shell-owned Pulau Bukom, and bacteria from topsoil used to reforest the Phase II site(Li, Ohtsubo, Higashi, Yamaoka, & Morishita, 2007); thus resulting in heavy metals possibly leaching into the seabed and the surrounding sea.

Also, while the infographic does substantially more information than what is available online, much of it is redundant. Being in the public lobby, one could assume the infographic is meant for the public. Personally, a technology showcase and a detailed characterisation of the Phase II lagoon water chemistry would have been more appropriate. Of course, this data is sensitive; but I do find it a missed opportunity to build trust with the public about the competency of local treatment facilities.

Sadly, when I tried to get in contact with Memiontec for an interview on these queries, I did not get a reply.

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Dung, T., Vassilieva, E., Swennen, R., & Cappuyns, V. (2018). Release of Trace Elements from Bottom Ash from Hazardous Waste Incinerators. Recycling, 3(3), 36.

Li, L. Y., Ohtsubo, M., Higashi, T., Yamaoka, S., & Morishita, T. (2007). Leachability of municipal solid waste ashes in simulated landfill conditions. Waste Management.

Lin, C. F., Wu, C. H., & Liu, Y. C. (2007). Long-term leaching test of incinerator bottom ash: Evaluation of Cu partition. Waste Management.

Manaia, C. M., Rocha, J., Scaccia, N., Marano, R., Radu, E., Biancullo, F., … Nunes, O. C. (2018). Antibiotic resistance in wastewater treatment plants: Tackling the black box. Environment International.

PUB (2016) Sludge Dewatering Used Water Treatment Process. Retrieved from:

Tchounwou, P. B., Yedjou, C. G., Patlolla, A. K., & Sutton, D. J. (2012). Heavy metal toxicity and the environment. EXS.

 Additional Notes

*  I got this piece of information from my NEA mentors during my internship in 2019, as well as through queries to NSL Oilchem Tuas 2’s site directors on a field trip.

 Incineration results in Fly Ash and Filter Cake that ends up in the flue gas filters too, but these are sent for further treatment before disposal on Pulau Semakau, according to an NEA guide on a Tuas Incineration Plant tour.

 This refers to Annex H of SS593 Code of Practice for Pollution Control on Page 42-43


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One Man’s Cure is Everybody’s Poison

Last week, we discussed the problems with antibiotics and how municipal wastewater treatment plants(WWTP) like ours worsen the spread of antibiotics, antibiotic-resistant genes(ARG) and antibiotic-resistant bacteria(ARB) into the environment.

This week, we expand our scope on pollutants to the subset of WHO’s persistent organic pollutants that encompass antibiotics, biocidal compounds, pharmaceuticals and endocrine disruptors; which I’ll term as bio-disruptive organic compounds (BOC) for the purpose of this post.

BES folks might recognise endocrine disruptors as an emerging pollutant in freshwater systems; but for the sake of those unaware, these BOCs alter hormone levels in organisms in a way that causes harm to the organism, its offspring, or those around them(Bergman, Heindel, Jobling, Kidd, & Zoeller, 2012; Godfray et al., 2019).

As the term “emerging pollutant” might allude to, new endocrine disruptors and BOCs are being discovered or manufactured(Bergman et al., 2012). Some of these originate from completely unintuitive sources: I mean, imagine flame retardants changing your hormone levels(Godfray et al., 2019)! I wouldn’t even have thought to look there for BOCs!

Not to mention, household products (especially plastics, glues, lubricating oils) contain plasticizers like phthalates, BPA and flame retardants like Polybrominated diphenyl ethers(PBDE) that are also endocrine disruptors(Monneret, 2017).

Yes…even I’m guilty.
Plastic containers, lubricants, and even vinyl flooring contain BOCs that can be washed into our sewers(Monneret, 2017).

Think about it, every time you wash your utensils, ride a public bus or wash your floor: we are washing BOCs down our sewers. With all due respect to PUB, our municipal WWTPs are still under-equipped to ensure complete and utter removal of all BOCs from our wastewater stream(Sui et al., 2015). I must stress utter removal because BOC concentrations of parts per trillion(ppt) are still harmful(Godfray et al., 2019).

Remember our 4 national taps? Yep, that same catchment water Sui et al (2015) found BOCs in bounces straight back at us. Of course, nothing is happening to our aquatic ecosystems or drinking water as of yet; but we can’t say with certainty that there will not be an ARB outbreak., an emergent BOC contamination, or any side effects of chronic exposure to BOCs

No point crying over spilt drugs, let’s talk solutions. The problem? Elevated concentrations of Diclofenac and Carbamazepine in ppt to ppb, and Caffeine in ppb to ppm; found around the Jurong Lake Catchment(Tran, Li, Hu, & Ong, 2014).

While there probably won’t be a universal solution to eliminate these BOCs from our wastewater, there is a particular pilot test that achieved significant results in a context analogous to Singapore’s WWTPs that I would like to present. A pilot plant utilising a novel technology, known as Membrane Distillation(MD), was implemented at Swedish WWTP as an alternative to an existing reverse osmosis array for the purpose of pharmaceutical wastewater treatment(Fortkamp et al., 2015). To put it simply, instead of using high pressures to squeeze water through a porous membrane, a selective hydrophilic membrane that only allows vaporised wastewater through is used.

The pilot managed to reduce Diclofenac, Carbamazepine, and Caffeine concentrations to below 3 ppt(Fortkamp et al., 2015). However, outstanding costs for heating wastewater was identified(Fortkamp et al., 2015). Fortunately, since PUB’s WWTPs recycles biogas for energy generation, a similar MD system with virtually no extra energy expenditure may be possible!

A possible opportunity to install a membrane distillation unit in our WWTPs. Source:

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Bergman, Å., Heindel, J., Jobling, S., Kidd, K., & Zoeller, R. T. (2012). State-of-the-science of endocrine disrupting chemicals, 2012. Toxicology Letters.

Fortkamp, U., Royen, H., Klingspor, M., Ekengren, Ö., Martin, A., & Woldemariam, D. M. (2015). Membrane Distillation pilot tests for different wastewaters.In IVL Report B 2236:…/B2236.pdf

Godfray, H. C. J., Stephens, A. E. A., Jepson, P. D., Jobling, S., Johnson, A. C., Matthiessen, P., … McLean, A. R. (2019). A restatement of the natural science evidence base on the effects of endocrine disrupting chemicals on wildlife. Proceedings of the Royal Society B: Biological Sciences.

Monneret, C. (2017). What is an endocrine disruptor? Comptes Rendus – Biologies.

Sui, Q., Cao, X., Lu, S., Zhao, W., Qiu, Z., & Yu, G. (2015). Occurrence, sources and fate of pharmaceuticals and personal care products in the groundwater: A review. Emerging Contaminants.

Tran, N. H., Li, J., Hu, J., & Ong, S. L. (2014). Occurrence and suitability of pharmaceuticals and personal care products as molecular markers for raw wastewater contamination in surface water and groundwater. Environmental Science and Pollution Research.

Additional Note:

If you would like to read more about Membrane Distillation, check out this book at National Libraries!

Specifically, pages 305-350. Really interesting stuff!


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