S5C9: Water Supply & Waste Treatment

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a) Origin and supply of water

In the first six chapter of this series, we have explored the most common means of transport crisscrossing our immediate environment and the globe at large, whether they move people or cargo. In Chapter 7 we then looked at the underlying transport infrastructure facilitating these activities and in Chapter 8 we discussed the processing of goods and passengers so they can enter, transit through and exit transport networks. This time around, we will cover most of these same aspects but in relation with the most important of all commodities in our life, that without which our bodies would not be able to maintain homeostasis and perish quickly: water.

We previously dedicated the entire S3 Section 2.d to the topic of the water cycle and this provides us with the starting point needed as it listed the different sources of water: harvested rainwater, stored freshwater in lakes and reservoirs, snowmelt, and groundwater that can be extracted from aquifers through wells. The most basic style of wells involves buckets lowered and brought back to the surface manually though, whenever economically feasible, this is replaced by an engine-powered pump system and, once extracted, the water is channelled through pipelines. We looked at both pump and pipeline technologies in S5 Section 1.e and S5 Section 7.f on aqueducts gave us a view of the engineering required to make up for the absence of motorized pumps.

Aqueducts relied on gravity and, if one thinks about it, we still do in many settings where water is stored in elevated tanks called water towers providing the required stored energy to distribute pressured water to residential areas without the need for pumps in each home. In a hilly neighbourhood, underground cisterns can replace water towers and still work with gravity, provided they are located at the top of the hill; otherwise a pump mechanism is required to bring water to the surface and channel it through pipes. If you happen to find yourself in Istanbul, there are several of those historical cisterns now transformed into tourist attractions in the Sultanahmet area.

The section on aqueducts also provided insights in the vital role of ensuring the provisioning of water and, nowadays, water is widely perceived as a future geopolitical flashpoint, an issue exacerbated by the increase in worldwide population, the relentless spread of pollution, and global warming which is making rainfall even more scarce in already arid regions. So, it seems that as we have seen in S5 Section 1.f, pipelines of all kinds are really at the core of geopolitical risks and tensions.

Water might be a commodity but just like many other commodities it may need refining to be fit for purpose. In this case, it requires treatment and possibly purification, we will look at these processes in section d). Consequently, there needs to be a separation between the flow and storage of water with different qualities. At the top of the list going by versatility is drinking water, meaning safe to drink without negative consequences, be they short or long term. We will look at the types of pollutants in water in section c). Depending on its quality or “grade”, which corresponds to the absence or not of pollutants and their nature, water can be segregated into different categories. For the sake of this chapter, we can trifurcate clean water into three of them: #1 drinking water, #2 fit for washing ourselves and flushing toilets, and #3 suitable for agricultural or industrial usage. It should be noted however that some specific industrial use may require highly purified water, or the removal of specific particulate matter. The point really, is to highlight that different circuits needs to be laid down and commingling should be avoided.

b) Waste water and sewage

Once water has been used, it needs to be treated before it can be released into the natural water cycle or reused after treatment. The degree of treatment required will depend on whether we are dealing with grey water, such as the one from the kitchen sink or your shower, black water containing faecal contamination, or industrial wastewater. The former can be partially treated before being reused for irrigation or flushing toilets for instance.

The channelling away of wastewater, what we call the sewage system, is not a new invention by any means but despite some warnings from Islamic scholars dating from as “early” as the 13^th century CE, it was not until the 19^th century that the role of water in carrying disease was fully appreciated and the concept of hygiene was fully formed to resemble how we appreciate it today.

The building of sewage is thus one of the first public or private works that needs to be undertaken when developing new residential areas as well as industrial and commercial ones. Conversely, the absence of a proper sewage system with treatment plants is a strong indication of a lack of regulation or a low level of economic development.

Sewage systems are generally split into two categories, depending on whether they also collect stormwater, in which case they are called combined sewers. Stormwater is water from heavy rains or melt that is not captured by the traditional rain harvesting or river channelling devices and piping. Hence, it runs overs surfaces that may be contaminated, including roofs and roads. So the reason combined sewers deserve to be thought of differently is because of the potential spikes in flow rate and the potential for water to contain additional contaminants. Unfortunately, in urban environments, little stormwater ends up being filtered through the soil and replenishing the aquifers and more than half of it ends up as runoff, whereas in a natural environment about half would end up in the soil, with fifty percent of that infiltrating deeply underground.

Where sanitary sewer remains separated from stormwater, this is the result of adequate management relying on a combination of dedicated drainage structure, artificial separators to remove the coarser solids from the water, and ponds or wetlands to serve as overflow reservoirs and decantation zones.

c) Pollutants in water

If we except junk solid waste that can be cleared with a crude mechanical filter (though this leaves the issue of disposing of this trash, something we will indirectly cover in section e), there are four types of pollutants in water: pathogens, nutrients, organic matter, and non-organic particles also called micro-pollutants.

Pathogens end up in water through human and other animal faeces, meaning they are being ejected out of one host organism, survive in the aqueous environment and can directly or indirectly make their way into another host. In that sense, water should be thought of as a medium with the potential of containing and transporting disease vectors. The most prevalent of all water borne diseases are gastroenteritis (also known as stomach flu), dysentery and the cholera while the most common symptoms resulting from water-mediated infections is diarrhoea, under all its forms and duration. Diarrhoea occurs when, as part of the digestive process, the large intestine is unable to reabsorb as much water and other liquids as it is supposed to under normal circumstances, thus resulting in loose and often watery bowel movements.

Moving on to nutrients and organic matter now. The former are problematic because runoffs of nitrogen and phosphorus can lead to eutrophication, a process involving the growth of algae shielding other plants from the sun rays they rely on and, after they die, aerobic decay will remove oxygen from the water, impacting other species in the ecosystem. The main culprits responsible for excessive nitrate, ammonia and phosphorus are the fertilizers used in agriculture. As for organic matter, it can be biodegradable or inert and it spans carbohydrates, proteins, pesticides, surfactants and aromatics. Some of these molecules might be toxic for certain species, including humans, or lead to the proliferation of microorganisms removing oxygen from the water for their own use or acting as disease vectors.

Finally, the category of micro-pollutants refers to artificial compounds, in particular small and microscopic plastic particles and pharmaceutical molecules. Once ingested by other organisms, these can easily prove toxic or, when they are not digestible or biodegradable, they can obstruct pathways within these organisms and lead to their untimely death.

d) Water treatment and purification

Depending on the provenance of wastewater and its intended use post-treatment, the type and number of treatments it will go through and the purification level required vary significantly. Below, I will provide an overview of some of the main processes that are commonly used. Time to get a little technical.

The primary step in water treatment is the removal of particulate matter and this may entail only some or all of the following processes:

Particles aggregation, be they organic or not. Having microparticles cluster makes it easier to filter them out. This can be done by adding chemical substances that will trigger a precipitation of dissolved molecules into a solid state or that will bond to certain suspended particles and deposit at the bottom of a tank, a phenomenon called flocculation. The former is achieved with coagulants neutralizing the electrostatic surface tension of molecules, which then agglomerate due to Van der Waals forces, a type of intermolecular interaction (refer to S1 Section 2.d for more info on this topic). The later relies on clarifying agents, typically positive ions tending to attract negatively charged particulates and to precipitate into flocs made of long hydroxide chains taking other particles along with them.
Sedimentation in a settling basin where the bottom part experiences little flow, allowing the floc to drop to the bottom under the effect of gravity and remain there until the resulting sludge is cleared out by specialized mechanical equipment. We can think of sedimentation as filtration through density differential.
Filtration of the clarified water to remove any remaining particle, including unsettled floc. The two main technologies are membrane and sand filters. Sand filtration combines the creation of a natural screening corresponding to the space between grains of sands with the exposure to materials such as activated carbon. The key property of such substance is its incredible surface area to volume ratio (one gram can have a surface area in excess of 500 m²) and its prior processing or activation so that it has strong adsorption characteristics – a link to the Wikipedia entry for activated carbon is provided at the end of this chapter if you wish to read more about this. This combination results in the adhesion of microparticles to the carbon and its filter-like behaviour. The purification level can be classified as follows: microfiltration removes particles down to 0.1 μm (1×10^-7m), ultrafiltration will use pore sizes of 0.01 μm and nanofiltration goes 10 times better at 1 nm (1×10^-9m). Beyond this level, pressure needs to be applied to create reverse osmosis and only water molecules will go through pore sizes of 0.1nm, making this the choice technology for purification to the standards required for drinking water. Left to its own, a solvent will move towards areas with comparatively high solute concentration and osmotic pressure is the threshold that needs to be overcome to retain the solute on one side of the pores while the now-purified solvent passes through the membrane. I include a link to the Wikipedia entry for reverse osmosis in section h) if the subject is of personal interest.

A last note on the above, before continuing on to disinfection, the second step. Ahead of coagulation and flocculation, the pH of the water may need to be adjusted if it proves to be too alkaline or acidic. This is necessary in making the water potable and this addition may also help in precipitating other substances already in the wastewater. Furthermore, a neutral pH reduces the propensity of metals to dissolve in water, a clear benefit in terms of safety since many pipes have metallic content.

The purpose of disinfection is the removal of pathogens, one of the pollutants identified in the previous section. Some of them will have been extracted during the previous treatments steps but the exposure to certain chemical substances, high temperature or radiations can kill or inactivate any remaining ones.

Pasteurization, the heating up to a temperature close to but below 100°C, is commonplace in the dairy and food processing industry to extend shelf life. Ultra-high temperature (UHT) processing adopts the same principle, pushing heating to 140°C for two to five seconds in order to sterilize liquid food.
Exposure to ultraviolet radiation will damage the genetic material of micro-organisms and kill or inactivate them. This has to do with the high energy contained within these radiations, which has a deleterious effect on any biological structure.
Chlorine is a strong oxidant and the addition of compounds containing this element will provoke redox reactions, destroying the lipid membranes of microorganisms and reacting with the intracellular proteins. Clearly a disastrous turn of events for them.
The addition of ozone (O₃) in water will also trigger oxidation because the molecule has a strong tendency to give up one of its oxygen atoms to become a stable diatomic oxygen molecule (O₂).

As a result of all this, we end up with treated water that can reach drinking standards. Somewhat ironically however, this is not a perfect situation since it is completely desalinated and demineralized and we traditionally rely on water to top up the concentration of electrolytes such as magnesium, sodium, chloride, potassium and calcium, which are so essential to many of our fundamental biologic processes; as we have seen throughout the second series on the human body and sensory systems.

e) Solid waste treatment

The sludge left over after the phase separation of sewage in a treatment plant through the sedimentation and filtering steps follows one of two trajectories, depending on whether it is hazardous or not. If hazardous, it must undergo further treatment until the final product is fit to be disposed of or reused, perhaps in an industrial context. Non-hazardous sludge waste on the other hand can be dumped at sea or in a landfill though, preferably, it can also experience further treatment including drying and composting before being reused as agricultural fertilizer.

The logic and processes are very similar for industrial waste; valuable chemicals can be recovered during the various waste treatment steps and either be reused or recycled into other products whereas the non-hazardous solids extracted from the water often find their way to a landfill or to an incinerator.

Municipal garbage collection follows the same route as these non-hazardous materials and the public service can be free or, in some municipalities, it can be priced according to the weight being disposed of, with the objective of incentivizing customers to: #1 purchase goods with less packaging or even in bulk, #2 sort the trash across various recycling categories, and #3 compost some of the organic waste for use as home-fertilizer.

This is part of a broader effort to reduce, reuse, recycle products as much as possible to extend their lifecycle. Not only does this reduce the volume of garbage but it also has an upstream impact by decreasing the demand for certain products and therefore the ecological and carbon footprint of the entire manufacturing process, including the raw material extraction and the transportation of the goods.

Personally, I am a massive advocate of “reduce” and act accordingly. Our policy as individual consumers should be to buy what we need, not what we want, and of course this entails being objective about the difference between the two.

At the bottom of the waste hierarchy, when all else fails, are incineration and landfills. Incinerators are plants where waste is burned while ensuring no noxious fumes escape. Technically, some of the materials undergo combustion, which is an exothermic redox reaction and the heat released increases the temperature of the steam and therefore the gas pressure which drives the electricity generation by the turbines – refer to S5 Section 1.a if you want to understand the workings of a steam engine better. The electricity is then sold and transferred to the broader regional grid and this is the end of the line in terms of recovery; the process results in ashes weighting 5 to 6 times less than the original waste.

Ignoring scavenging, there is no recovery taking place in a landfill. The idea is to find a stable site and avoid infiltration of liquids into the ground by using thick impermeable liners. The garbage brought there is then compacted before being dumped and undergoing natural decomposition after which it is sealed and the land can then be repurposed after being covered with soil and redeveloped to welcome industrial, commercial and residential structures, noting that those may require deep or special foundations due to the relative instability of this ground.

The natural decomposition process can be broken down in five phases, including acid formation which can accumulate in the leachate in high concentration, hence the risk of groundwater contamination if the lining is not fit for purpose, and methane formation, which precludes the proximity of any activity that could lead to ignition.

f) Radioactive waste management

Of all the types of waste our society deals with, none seems as problematic as the radioactive kind, at least from a technical standpoint because one could rightly argue the sheer volume of industrial and consumer waste, including the plastic plaguing the ocean ecosystems, is actually more consequential.

There are two fundamental issues with radioactive materials: they are hazardous to biological life above a certain threshold, especially the high-energy kind, and they can remain above this threshold for a very, very long time. Radioactivity is dangerous because it produces ionizing radiation such as alpha particles or the more penetrating gamma rays, and these can cause damage to the tissue and genetic material of living organisms.

Beside naturally occurring radioactive materials, including some within the mined rare earth ore processed and shipped for industrial use, the vast majority of radioactive waste is spent nuclear fuel linked to nuclear power generation and from nuclear weapons production and reprocessing.

The reprocessing of spent nuclear fuel is an expensive, complex separation and purification process of the fission products and the metallic actinides (including thorium, uranium and plutonium). Beyond the advantage of yielding reusable elements, this practice also helps reduce the total volume of radioactive waste. I include a link to the Wikipedia entry for nuclear reprocessing in the next section, if you are interested to read more about this.

To avoid leaching and contamination of ground water and more broadly any type of reaction or degradation, the hazardous waste is either vitrified or injected into a crystalline ceramic host form. The waste is then disposed of depending on its classification, which is a function of the half-life of its radioactive material and the nature of the radiation, as mentioned earlier. Low-level waste is material that may have been exposed to radiations but is not thought to be genuinely hazardous and it is segregated from standard landfills with precautionary extra physical separation and containment. Intermediate-level waste requires shielding and high-level waste needs both shielding and cooling.

Considering the half-life of some elements can extend not simply to decades but to thousands or hundreds of thousands of years for certain elements, long term disposal solutions have to be devised – you may refer to S1 Section 7.c on radioactive decay to better understand the concept of half-life. There is a consensus among scientists, and even politicians it seems, that the best method is to bury them in deep geological formations. Far from sight and the surface. The most advanced project is Onkalo in Finland and other countries are planning their own – it’s about time considering nuclear power generation has been a thing since the 1950s and has taken off post the 1973 oil crisis when the price of fossil fuels climbed steeply.

g) Trivia – Air quality index

Pollutants are those substances considered to be harmful to humans, other living organisms and our environment. The main gaseous pollutants our technological society needs to deal with are ozone, sulphur dioxide, carbon monoxide and nitrogen dioxide. Moreover, to this list of noxious gases, we must add airborne particulate matters, which may include toxic substances such as lead from vehicle exhausts. They are mostly problematic because they can penetrate in our lungs and even cross into our bloodstream. This means there is no safe level of exposure or rather ingestion of particulate matter, and the smaller the size the more it can diffuse throughout our body. For reference, PM 10 refers to size under 10 microns and for PM 2.5 it is below 2.5 microns.

Air Quality Index (AQI) is a measurement of air quality based on certain thresholds and, in some jurisdictions, the concentration they refer to is not computed linearly and give the appearance of under-reporting the real level. Nor are the thresholds and algorithms the same across countries. These thresholds exist both to warn and advise the public and, in some cases, to enable the trigger of regulations to lower the volume of pollution-producing activities, including car traffic in congested urban settings.

At the receiving end, individuals can protect themselves by restricting their exposure to the outside air and running air purifiers. Some of the filtering mechanisms of these devices are similar to those we have seen in section d) on water treatment, including the use of activated carbon. The other alternative, when going outside, is to wear a mask with suitable filtering capability, such as the N95 respirator used in regions faced with the seasonal haze produced by forest fires.