Municipal heart attack
What happens when a city’s sewerage system gets a heart attack? Ridiculous proposition? Not really if you consider the amounts of fats, oils and grease (FOG as it is called) entering sewers nowadays. The global obesity problem is linked to it.Says Worldwatch magazine, “The rise of the fast food industry and the growing number of people dining out have had consequences for human and urban fat-processing systems alike, and there are ironic parallels between the metabolism of fat in human bodies and the capacity (or incapacity) of city waste systems to handle the increasing volumes of fat wastes”.
Around the world, sewer blockages and overflows are becoming more frequent because restaurants, bars and cafeterias are pouring their cooking residues into drains. Worldwatch says the volume of restaurant grease alone has reached 1.4 billion kg in the US. It would be interesting to have the figures for Asian cities.
Wrote Barry Newman in the Wall Street Journal: “Fat won’t pollute; it won’t corrode or explode. It accretes. Sewer rats love sewer fat. Solids stick in fat. Slowly, pipes occlude. Sewage backs up into basements — or worse, the fat hardens, a chunk breaks off and rides down the pipe until it jams in the machinery of an underground floodgate. That, to use a more digestible metaphor, causes a municipal heart attack.”
Restaurants, apartment complexes and commercial buildings often install grease traps or interceptors, however these need to be properly designed and constantly maintained.
Finding blockages in sewers has always been a challenge for local authorities. Techniques such as CC TV, smoke infrared thermography, radar and sonic technology have been developed but these come with a price tag. High-pressure hoses can push and dislodge FOG but these cause problems downstream. Large vacuum trucks can also be used to suck them out from congested sewers.
Solvents, caustics, and acids may dissolve FOG and transport them out, but being toxic, they can have harmful effects on the treatment system and its employees. Enzymes and detergents pose other problems.
Animal and vegetable FOG is degradeable by wastewater microorganisms, but treatment is not easy. Due to the high BOD levels, the treatment plant must have an aeration system that is capable of supplying the additional oxygen necessary. A longer solids detention time is also required; and there will be a greater biomass yield. In an activated sludge system FOG will contribute to the growth of filamentous bacteria. As fats are broken down, they will adsorb to the floc, making it more buoyant and more prone to clarifier washout. Because of these characteristics, treatment in the aerobic digester is the better choice, provided capacity is available. Anaerobic digestion is also an option, although there is the possibility of increasing the scum layer formation and foaming.
The easiest way to deal with this problem is to not let the fats enter sewers in the first place. Public education is the foremost step. Fats, oils and greases should not be poured down sink drains or flushed in toilets. Most people are unaware that grease sticks to the insides of sewer pipes and over time, builds up to block the entire pipe. The urban public needs to be informed about the need to scrape grease and food scraps into a can or the trash for disposal (or recycling where available).
Our lifestyle is reflected in our sewers. What’s good for our hearts will probably be good for our sewers too. Now where’s my low-fat cookbook?