In developed countries, the sight of garbage is put behind a rather expensive veil, in our part of the world, it’s part of our daily existence.
Insert doodle – Karachi garbage
When we moved to Karachi, it wasn’t hard to believe that the city alone produced over six thousand tons of solid waste every day – it was in plain sight. The deplorable condition of garbage-strewn roadsides and streets around the city began to really bother us.
There came a point when we felt utterly frustrated by the situation in which the municipality was left completely handicapped by the warped political dynamics of the city. We became whiners, complaining incessantly – so much so, that we had to seriously tell ourselves to either do something about it or forever hold our peace!
Having had experience in creative design, our first ideas drifted toward a design-based solution but we realized that this problem required a transdisciplinary approach, for which we needed to educate ourselves first. In the spirit of learning, we began searching for solutions for this complex problem in which sorting of waste was not an option and simplicity in design and function was the first priority. It took us years investigating urban waste at dumpsites – working our way up from a secondhand steel kettle to a seven foot tall, stainless steel thermal depolymerizing unit, while drawing inspiration from Vladimir Shukhov’s ideas on thermal cracking.
With continuous experiments, we built further on the Shukhov cracking process by introducing a state of partial vacuum, and combining it with a valveless Pulsejet engine. The unit that resulted is responsible for producing the heat required for an endothermic process of thermal depolymerization. The Pulsejet engine design used in this system is a Chinese innovation that has removed the only moving part in this engine, making it even simpler.
We designed and fabricated a unit with three main chambers
- A burner (Pulsejet engine)
- A thermal cracking chamber (anaerobic condition in a state of partial vacuum)
- A thermal activation chamber (high pressure fuel line)
The process of thermal cracking in a state of partial vacuum expedites degassing, lowers the melting point and increases the rate of dehydration – all these conditions favor decomposition. Pulsejet on the other hand proves to be an efficient fuel combustion system, owing to its ability to naturally aspire and operate at subsonic frequencies without any active assistance. The task at hand was to achieve a temperature range between 450 and 475 degree centigrade, under reduced pressure, for anaerobic thermal decomposition of organic matter and plastic waste.
This unit and its next iteration, as seen in the pictures above, can be best understood as an interplay of temperature and pressure. The function of the thermal activation chamber is that of a fuel line that continuously produces fuel for the engine. This is achieved by feeding it garbage from dumpsites. Once operational at optimum temperature, the fuel production cycle kicks in and the system becomes standalone, not requiring any source of conventional fuel. Unlike the decomposition chamber, here, the pressure and temperature are kept extremely high. High pressure does not allow gasification of the contents in the fuel line while the temperature is maintained higher than the combustion temperature of the contents. This highly charged up liquid when sprayed in air through an atomizer, catches fire spontaneously. This fuel oil/ gas mixture consisting primarily of hydrogen, carbon monoxide, and some carbon dioxide is called synthetic crude/syngas which can be re-blended as solvents. Currently, the most desirable product that can be formed from syngas is ethanol.
The first iteration successfully performed the required process but mild steel used in fabrication proved inadequate in resisting erosion due to the high temperatures, especially the engine casing. The second iteration took into account the metallurgical requirements, the entire casing was fabricated in stainless steel.
We had set out to find a way to solve the municipal solid waste problem in Karachi, our resolve was to convert solid waste into synthetic fuel oil/ gas. Although we managed to achieve the desired results in our experiments, we became increasingly apprehensive about the degradation in the environment around dumpsites which were our primary sites for installation. It seemed as though our solution ended up creating just another problem – in reality we were far from a workable resolution.
This failure and the resulting lessons learnt call for a third iteration that is yet to be prototyped and tested. Our focus now is on a cleaner source for heat generation in the decomposing process – for obvious reasons, we’re turning to solar energy! Changing the heat source means changing the entire system and we’re thinking, while we’re at it, we could also decentralize the process to achieve feasibility. A centralized solar treatment unit would still require municipal solid waste to be collected from the city and brought to a site – the scale of such an installation would be massive. If we take account of the amount of sunlight every household receives against the waste they produce, the prospects are promising because each household can generate enough energy to treat all the waste that its occupants produce.
The system works on a partnership where the municipality installs an underground hewed garbage/sewage sludge storage tank, an underground fuel oil/gas collecting tank, a simple network of pipes, a solar powered vacuum pump and solar hotspots on the rooftop of a partner house for thermal conversion of the waste produced by the household, into fuel oil/gas. The process consists of a cycle involving a specialized municipality truck designed to have the following compartments:
- A garbage hewing chamber
- A sewage sludge holding chamber
- A hewed garbage/sewage sludge mixing chamber
- A fuel oil/gas holding tank
The process starts with the arrival of the municipality truck that collects solid waste from the partner house
- It then pumps a mix of hewed garbage/sewage sludge into the underground storage tank of the house through a pipe
- This mixture is sucked up to the roof through pipes by the solar powered vacuum pump
- As the mix makes its way to the roof, it’s exposed to high temperatures while running through a network of pipe and passing through a series of solar hotspots
- The mix first dehydrates and then thermally decomposes into fuel oil/gas.
- The fuel oil/gas is collected in the underground collecting tank through a return pipe, leaving behind carbon in the form of char, a by-product that has multiple uses
- The cycle completes when the truck extracts fuel oil/gas from the underground oil/gas collecting tank into it’s holding tank.
The truck repeats the process every 24 hours, going to multiple partner houses each day. This mechanism may have an edge over many other prevalent systems because the garbage does not get accumulated, instead it is collected and converted into a commodity on the go. Any such garbage truck would end its day’s routine with a load of synthetic fuel in its holding tank, rather than a heap of garbage to be disposed.
Read more on our material explorations by following the links below