What must never go down the drain

Oils, paints, solvents, medicines, batteries and collection points. Theory, formulas, worked examples and applications.

The drain carries the problem to someone else

Not everything that fits down the drain should go through it

A household drain or a toilet can physically swallow almost any liquid or small solid poured into it, but that physical capacity to swallow it does not mean at all that the discharge is safe or that it will disappear without consequences. Used cooking oil, as it cools, forms solid deposits that progressively clog pipes and also hinder the wastewater's own biological treatment process, because the floating grease layer interferes with the oxygen the treating microorganisms need. Used motor oil contains heavy metals and other contaminants concentrated by the engine's own use, and always requires separate specific collection, never the household drain. Paint, solvents and concentrated cleaning products can be toxic to aquatic life, flammable, or both at once. Expired or leftover medicines, if flushed down the toilet, alter living organisms present in the water even at very low concentrations, precisely because many of them were designed to have a potent biological effect at small doses. And batteries, if mixed without protection with other waste, can cause fires from short-circuiting, on top of definitively losing metals that could actually be recovered through a proper collection circuit.

A treatment plant is not a universal chemical-waste facility

A wastewater treatment plant is designed, from its original conception, to treat the typical composition of normal domestic wastewater — organic matter, certain dissolved nutrients, common pathogens — through biological processes that depend on carefully maintained populations of living microorganisms. It is not designed, however, to safely and completely process any arbitrary chemical substance someone decides to pour down a household drain, and many hazardous substances can, in fact, damage or even kill those same microorganism populations the entire plant's correct operation depends on. There is also an important physical paradox in managing these discharges: the more a specific hazardous substance is diluted in more water before being discharged, the greater the total volume of contaminated water that then has to be managed, and the harder it becomes, technically and economically, to recover that substance in isolation once already diluted — diluting a problem never solves it, it only disperses it and often makes it more costly to treat properly.

Managing well at home, before the waste leaves home

Always keeping a household chemical product's original label makes it much easier to correctly identify its composition and proper handling in case of doubt or accident. Never mixing different products together avoids unforeseen chemical reactions that can generate toxic gases or dangerous heat. Buying and using only the amount of product actually needed reduces how much hazardous leftover ends up accumulating at home for years. And taking each specific hazardous waste to the local channel specifically designed to manage it — a municipal recycling center, a used-oil collection point, a pharmacy for expired medicines, or the retailer itself for certain electronic waste and batteries — is always the right option, instead of improvising unsafe home management. A swollen, damaged or visibly deteriorated battery also requires following the specific professional instructions for safe handling and thermal isolation, and should never be sent by ordinary mail or handled without proper protection.

The buying decision is already deciding the future waste

Choosing small package sizes when only a small amount of a product will actually be used directly reduces how much hazardous leftover will need managing afterwards. Choosing, when a functionally equivalent alternative exists, a product with lower intrinsic hazard reduces the risk tied to its entire subsequent management chain, from storage at home to final treatment. And choosing repairable products and devices, with an easily replaceable battery, reduces how often that specific hazardous waste — a spent battery, for instance — has to be generated in the first place. Preventing a hazardous leftover from ever coming into existence is, in practice, a much safer and simpler decision than having to store it correctly at home for years while waiting for proper management.

Buying also decides the waste

  • Never pour cooking oil, motor oil, paint, solvents or medicines down the drain or toilet: a treatment plant is not designed to process them.
  • Keep original labels and never mix products together; it avoids unforeseen chemical reactions.
  • Use only the amount of product you need to reduce hazardous leftovers accumulating at home.
  • Take each waste to its specific channel: a recycling center, a used-oil collection point, a pharmacy for medicines, or the retailer for batteries and electronics.
  • A swollen or damaged battery requires professional handling; never send it by ordinary mail or force it.

The purchase already decides the future waste

Choose small packages if you'll use little, lower-hazard options when they work just as well, and repairable products with a replaceable battery. Preventing a leftover from existing is safer than storing it properly for years.

The numbers, with their source

A single oil change can contaminate a million gallons of water

The used oil from a single motor-oil change can contaminate up to one million gallons of fresh water — a year's supply for about 50 people — according to the US EPA. At the country scale, the EPA estimates about 143 million gallons of used oil end up contaminating the environment every year, equivalent to 14 Exxon Valdez oil spills; on top of that, more than 200 million additional gallons of used oil generated by individuals doing their own vehicle maintenance end up in the trash, in water, or poured directly onto the ground.

Source: EPA — Managing, Reusing, and Recycling Used Oil.

Why diluting a hazardous waste doesn't solve it

Used motor oil is an insoluble, persistent substance that can contain toxic heavy metals such as zinc, lead and cadmium, capable of affecting wildlife, vegetation, surface water and drinking water if not managed correctly. Most used-oil contamination, according to the EPA itself, comes precisely from small individual generators — people doing their own vehicle maintenance — who pour the oil around a fence, a tree, or directly down a street drain, not from large industrial facilities.

Source: EPA — Used Oil Management Program.

What this table is missing

These figures refer specifically to used motor oil in the United States; other household hazardous waste — paint, solvents, medicines, batteries — has its own scale of impact and its own specific collection channel, which varies considerably by country and specific municipality.

Sources