Among the 30 articles on the April 22 Town Meeting warrant, voters will be asked not only to support a $26 million upgrade of Middleborough’s 33-year-old wastewater treatment plant, but to spend $1.8 million to $3 million for the design. Environmental Partners of Quincy is working with the town to ensure that the plant complies with new federal standards. The plant is currently in compliance, but won’t meet new requirements, officials said at a meeting on Monday. Included in the fixes would be automated operations as well as new roofs, windows, and doors, ventilation and air conditioning, and fire protection. A 1,500-square-foot addition is planned for office, meeting, and storage space, as well as accessible bathrooms and a locker room for staff.