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Revision as of 07:01, 27 October 2017
Identifying the problem
Ammonia formed due to hydrolysis of urine is a major cause of the stench that is characteristic of public toilets. Apart from putting off users, ammonia is also a health hazard.
Ammonia is irritating and corrosive. Exposure to high concentrations of ammonia in air causes immediate burning of the nose, throat and respiratory tract. This can cause bronchiolar and alveolar edema, and airway destruction resulting in respiratory distress or failure. Inhalation of lower concentrations can cause coughing, and nose and throat irritation. Ammonia’s odor provides adequate early warning of its presence, but ammonia also causes olfactory fatigue or adaptation, reducing awareness of one's prolonged exposure at low concentrations.
Apart from users, toilet janitors are exposed to ammonia for prolonged periods of time. In crowded localities, residents and shopkeepers in the vicinity of public toilets are also exposed to toxic ammonia fumes throughout the day.
Existing Solutions
Different solutions exist to tackle the problem of stinking toilets. The first involves using deodorants, which simply mask the odor. This is obviously not a solution as such, as the toxic fumes are merely disguised and wished away!
The second solution is to use an exhaust fan, which sucks away odors to the outside atmosphere. This also is not a solution; the ammonia is simply diluted into the atmosphere.
The third solution is using copious amounts of water to clean toilets. While this is the most obvious way to prevent formation of ammonia, it is not the most feasible due to scarcity or even unavailability of water, especially after a poor monsoon. A greater menace is that of unscrupulous persons peeing in public places!
The fourth solution is to construct waterless toilets. A version of waterless urinals employs a propriety liquid (usually oil) in the drain. The oil floats on urine, and odors from urine cannot rise from the oil layer. This is not a cheap solution, and also requires periodic flushing with lots of water.
A fifth solution is to use microbes (Bioblocks®) that degrade the urea in urine, thus preventing ammonia formation. This is by far the neatest solution, as ammonia does not form at all. However, the cost of replacing the microbes in the drainage system is a big impediment to its widespread use.
Our Solution
We decided to tackle the problem of stench without reconfiguring existing toilets, which would not be feasible because of the costs involved and the need to convince a number of stakeholders. We also decided that the solution should be agnostic to people’s attitudes and their disdain for cleanliness!
This, then, is what we propose: the ammonia in air can be assimilated by bacteria, which will use it as their sole source of nitrogen. The bacteria will metabolize the nitrogen to form a blue-colored compound that can be extracted and used as a dye, thus capturing the ammonia in an inert form.
We decided to engineer Escherichia coli to carry out the task of ammonia assimilation.
Mechanism
Ammonia in air dissolves in water to form ammonium, which enters the cell, condenses with glutamate to form glutamine due to the action of the endogenous enzyme glutamine synthetase (GS, encoded by glnA). Glutamine can be acted upon by indigoidine synthetase (encoded by indC from Photorhabdus luminescens) to form the blue pigment indigoidine.
Protons released during glutamine formation can be pumped outside the cell by proteorhodopsin (from uncultured marine Gammaproteobacteria of the SAR86 group). The proton gradient thus formed can drive endogenous ATP synthase to form ATP, which is required for GS activity.
