TREATMENT AND RECYCLING OF ARSENIC LADEN ALUM RESIDUE: A CASE HISTORY

The Metropolitan Water District of Southern California (MWD) owns and operates five water treatment plants, which collectively supply 60 percent of the drinking water distributed throughout the Southern California region. One of these plants is the Joseph Jensen Filtration Plant (Jensen Plant) located in Granada Hills, California. Water is normally supplied to the Jensen Plant from Northern California via the State Water Project's California Aqueduct. This Aqueduct is connected to the head of the plant via a 20-foot diameter section of underground line, known as the Newhall Tunnel. The Jensen Plant is also connected to the Los Angeles Department of Water and Power (LADWP) Aqueduct which can provide a secondary water source if needed. The LADWP Aqueduct carries water from the Eastern Sierra Nevada mountain range to a nearby DWP plant. Naturally occurring arsenic is found in both water sources. However, arsenic levels in the LADWP source are significantly higher than those found in the State Water Project source.In early 1995, the Newhall Tunnel was closed for 2-1/2 months to repair previous earthquake damage. During this period, the Jensen Plant received water from the LADWP Aqueduct, which contained the higher arsenic levels. As a result, the alum residue, generated during this time frame, contained higher levels of arsenic than expected. These arsenic levels exceeded the allowable sewer discharge limit and were approaching California regulatory levels as the residue settled and thickened. Consequently, the Jensen Plant was faced with an acute residuals management issue that it had not previously encountered.The issue for the Jensen Plant was how to properly remove and dispose of the arsenic affected alum residue that they had generated. In order of priority, the principal concerns, which guided the resolution of this issue, were; a) regulatory compliance, b) time, c) environmental liability and d) cost.This paper will examine the decision-making process and summarize the operational method used to dispose of the arsenic affected alum residue, with the stated concerns in mind.