If the mixed liquor color has changed from brown to black, what action should be taken?

When the mixed liquor color changes from brown to black, this typically indicates a significant increase in the concentration of sludge, often pointing to a problem such as the presence of excessive waste activated sludge (WAS). A black color in the mixed liquor can suggest that the microbial community is being overwhelmed or that there are insufficient dissolved oxygen levels for proper treatment.

Increasing the Waste Activated Sludge (WAS) is the appropriate action because it helps to remove excess biomass from the system, which can restore balance and improve the efficiency of the treatment process. By doing so, the conditions can return to a more favorable state where microbial growth can be managed effectively, thus enhancing the overall treatment performance and preventing potential issues such as foaming or poor settling characteristics.

Maintaining current conditions may lead to continued degradation of mixed liquor quality, while decreasing WAS could exacerbate the problem by allowing for further accumulation of sludge. Increasing aeration time can improve oxygen levels but may not directly address the excess sludge issue, which is better managed through increasing WAS removal.