The wastewater company will install the system, possibly under the supervision of your site assessor to ensure compliance with the planning conditions as set out in your grant of approval.
A digger will in all likelihood be required, to dig trenches to connect the pipes from the house to the septic tank and to dig a hole for the septic tanks and other treatment systems including the percolation area. Check in advance who is to arrange for the digger and who will operate it. A groundwork contractor is likely to brought on board.