PORTLAND — The Portland Water District’s Board of Trustees on Monday unanimously approved a $37.4 million operating budget and a $9 million capital improvement plan for 2011.

The operating budget increased by 0.6 percent over the current year. It assumes a 2 percent increase for residential users beginning in mid-2011, which would amount to a 46 cent increase on the average monthly water bill. The board will determine rates in the coming months.

The wastewater service fees that the water district charges municipalities will remain steady at 2010 rates. Local municipalities determine the wastewater rates for residential users.

The capital improvement plan allocates $1 million for a pilot study and engineering design of an ultraviolet treatment system, $2.2 million for upgrades of wastewater facilities and $4.6 million in water distribution system renewals.