What septic tank to choose for clay soil for the summer?

Good afternoon. Depends on several criteria:
- the number of people living simultaneously in the country. The norm / day for one taken 200 liters. It is desirable to consider with a margin;
- in your case, due to the proximity of groundwater, the model should be horizontal;
- choose a gravity-fed system, it will cost less than a forced one and is fully suitable for summer use;
- It is important to pay attention to the degree of purification specifically in your case. Clay soil requires you to choose a modification with a degree of purification of at least 95%.

You can study the issue in more detail in an article on our website. https://engineer.techinfus.com/en/kanaliz/septik/kak-vybrat-septik-dlya-dachi.html.