I mix biowaste with mixed waste (ends up to a landfill). In my household the amount of produced biowaste is minimal, mostly consisting of leftover pasta and fruit & vegetable peels. Will I cause harm to the environment by not sorting biowaste?
It depends a bit how your waste processing facility handles the 'mixed waste' stream. In the best case scenario they sort out everything and your biodegradable waste ends up where it belongs after all; on a compost heap or in a biodigester where it is turned into compost and/or biogas. However this is very unlikely and chances are that the mixed waste is either landfilled or incinerated. In both cases it is a waste of resources.
Organic waste that is landfilled takes up valuable space, but what's worse it will emit methane when it degrades due to lack of oxygen. Methane is a very potent greenhouse gas and contributes to global warming.
Waste that is incinerated is often used to generate heat and/or electricity, so there could be a small benefit here. However there are much better and more environmentally friendly ways to generate heat and electricity Also by burning organic waste you do not have the benefit of recycling it into compost. This means a bigger dependency on artificial fertilizers.
In the end, the small amount of biowaste you dispose will have very little influence on the environment, just like the emissions from only your car have very little influence. The trouble is that there are probably millions of people who dispose waste this way (and billions that drive a car). If all people continue doing what they do, nothing will change and the environment will continue to degrade. By altering your behavior you can set an example for other people and the change begins.