In wastewater treatment, what typically leads to lower DO levels in a receiving stream?

The presence of organic materials in a receiving stream typically leads to lower dissolved oxygen (DO) levels due to the process of biochemical oxygen demand (BOD). When organic matter such as plant debris or wastewater is introduced into a stream, microorganisms decompose this organic material. This decomposition process consumes oxygen from the water, thereby reducing the level of dissolved oxygen available for aquatic life.

Higher levels of organic materials in the water result in more microbial activity, which intensifies the oxygen demand. If the demand for oxygen exceeds the natural rate of oxygen replenishment in the stream (through processes like diffusion from the atmosphere or photosynthesis), the DO levels will drop significantly, potentially harming aquatic organisms that rely on adequate oxygen levels for survival.

It's important to note that other factors like water movement, temperature, and sunlight can also influence DO levels, but they do so in different ways. For example, increased water movement generally helps to aerate the water and increase DO levels, while high temperatures can lead to reduced solubility of oxygen. Lack of sunlight can impact photosynthetic oxygen production, but the primary driver in this scenario of lowered DO is the decomposition of organic materials.