Following compounds may be disposed by flushing them down the drain if there are only small quantities:
- Salts of light metals like calcium chloride or sodium sulfate
- Mineral acids (Hydrochloric acid, sulfuric acid, phosphoric acid)
- Sodium hydroxide, potassium hydroxide
But what is a "small quantity"? Nobody will answer this question. Everybody knows that small quantities make less problems for the environment. But since nobody can quantify the problems for the environment you will in most cases get the answer: "In case of doubt it is better to dispose as toxic waste." The more clueless your counsel the smaller are the tolerable quantities since every counsel fears to give a wrong advice which could be a criminal act at the end.
You will find yourself in surroundings which may be even hysterical and you will have to make your own decision. You will have no exact criteria but only your gut instinct instead. What kind of waste is it which you would accept as a "small quantity"?
- How toxic is it for the environment?
- Is it degradable or will it persist forever?
- Is it a rare waste or are all of your collegues producing the same kind of waste day by day - of course everybody only small quantities?
- Is there a didactical motivation to educate the students to have an integral view on the problems?
With mercury you will fight for every milligram not to escape with the waste water. With sodium sulfate you will be much more generous. Consider all the implications if you dispose as toxic waste. For example sodium sulfate will be burned and on heating it to 1200 °C it will develop sulfur trioxide which will escape with the exhaust gas and therefore has to be filtered off.
OK - you may produce gypsum boards with the filtered material but the slag has to be placed into a subsurface repository. If you compare this with disposing the sodium sulfate with the waste water, reflect if the salting of the water is not the minor problem.
According to local regulations (Indirekteinleiterverordnung) there are maximum concentrations defined for several compounds, particularly for some heavy metal salts like mercury but also for copper. The specific values will not help you because you do not know the concentration of the waste water of the whole institute building when it is released to the sewerage water system. For all the compounds listed in the "Indirekteinleiterverordnung" this is only a hint for you: "Be careful!"
If your "small quantity" is not a problematic "small quantity" it may be a help for you that this is often called "small quantity typical for use in a lab". This means quantities up to one kilogram.