By how much can you reduce the square footage of a sand filter if it is preceded by an ATU?

The correct answer reflects that when a sand filter is preceded by an Aerobic Treatment Unit (ATU), it can effectively reduce the square footage required for the sand filter by half. This reduction occurs because an ATU enhances the treatment of wastewater by introducing oxygen, which aids in the breakdown of organic matter, significantly improving the effluent quality before it reaches the sand filter.

With a higher quality effluent from the ATU, the sand filter is tasked with further treatment of less pollutant-laden water, resulting in a smaller footprint necessary to achieve the desired level of purification. The effectiveness of the ATU allows for a more efficient filtration process, therefore drastically reducing the area needed compared to a conventional system that doesn't utilize this advanced treatment step.

In scenarios where a sand filter operates without an ATU, it would need to accommodate for a greater volume of waste and potential contaminants, leading to an increased size requirement to handle the filtration effectively. The introduction of the ATU optimizes the overall system performance, making it possible to reduce the size of the sand filter significantly while still ensuring it performs its function adequately.