I'm aware that variants of this question have been discussed before (such as "how to build a self-sustaining medieval city?"), but my question comes at the problem from a slightly different angle.
Imagine that you are tasked with taking around 10,000 people, and imprisoning them within an enclosed city-space, from which they cannot get out, and nothing can come in (how that is to be accomplished is not relevant to the question). In other words, the isolation of the city is absolute, and after you've done your job once, even you can't intervene any further.
The only condition is that the people or their descendants should not die out - i.e., the enclosed city must be able to sustain itself at a basic level, and for a couple of thousand years. You belong to a rather high-technology civilisation set in the distant future, and so you have a near-God-like freedom in designing the elements of the city - the location (for weather and climate), the soil, availability of water etc. You cannot build anything specific for these people - all of that they are meant to do for themselves.
The people in question are, at present, at the same technological level as you, but there is an expectation that forced isolation will, over generations, lead to a sharp regression.
I've studied the previous discussions on similar topics, and I have a pretty good idea of the ratio of farmland to population (and other similar considerations) when it comes to the question of food. What I'm specifically concerned about is (a) designing a sewage system, (b) sources of clothing, and (c) the disposal of dead bodies. Any suggestions on those counts - and of course, any element I'm missing out in ensuring that the city survives - would be most welcome.
Edit: A few clarifications in response to the questions below. By "enclosed", I am only referring to a physical enclosure - say, by an impenetrable wall, or, as someone suggested below, a force-field. Sunlight etc, in the normal way, is all available.
One exception to the "enclosed" rule is the existence of a river, that runs through the city, and passes underground (and ultimately beyond the barrier between the city and the rest of the world). The source of the river is not known (to the inhabitants), and the dominant explanation for its existence is (obviously) a religious one.