Assessment of oxygen demand loads and its application in effective decision-making for the rehabilitation of an urban lake

Abstract

Urban lakes grapple with degradation, primarily due to excessive oxygen demand (OD) caused by loads of organic matter (OM) and nutrients. An OD load balance was developed to assess conditions and propose necessary actions to reduce pollutant loads, thereby improving water quality in Lake Nabor Carrillo, Mexico. The external and extracted loads of OD were determined based on flow rates and pollutant concentrations; the internal load was obtained through the determination of gross primary productivity; the load of OD accumulated in sediment (OD_sed) was calculated based on the fraction of OM in sediment relative to the total incoming OM; the mineralized loads of OD in water and sediment were determined through respirometry; and the load of dissolved oxygen (DO) was obtained by applying the load balance for steady state. The final OD in water was determined through load balancing. By modifying parametric values in the balance, the final DO was calculated for different rehabilitation scenarios aiming to achieve sufficient oxygen for protection of aquatic organisms. The current oxygen demand is 1824 t yr⁻¹ in the lake. To achieve saturated dissolved oxygen, tailored measures beyond standard environmental regulations for discharges are needed by phosphorus (TP) removal to a concentration of 1.95 mg L⁻¹. Alternatively, reducing BOD and TP to the maximum permissible limits for treated water into a lentic water body, along with applying 860 t yr⁻¹ of oxygen, can also achieve the goal. Both scenarios would require five years to achieve stable results through the application of the proposed technologies.