WATER quality at almost one in five of Hongkong's beaches deteriorated last year, a report has revealed. A study by the Environmental Protection Department (EPD) showed concentrations of the bacteria Escherichia coli had increased at 11 of the 56 beaches monitored. The density of the bacteria indicates the extent to which water is contaminated by faeces. The department's principal environmental protection officer for water policy, Mr Malcolm Broom, said beach water quality would not greatly improve for the next two or three years. ''We're not expecting any significant improvements before the 1995 bathing season when various sewage projects, such as the one in Castle Peak, should be completed,'' he said. Mr Broom attributed the deterioration in water quality last year mainly to heavy rains that flushed pollutants accumulated in ditches and nullahs out into the sea. He said water quality could improve slightly this year if the territory enjoyed hot, dry weather, which caused bacteria to decay quickly. Another summer marred by torrential downpours could cause worsened conditions for swimmers. The Urban Council opened its 11 beaches for this year's swimming season on March 1, while the Regional Council followed suit at its 28 beaches on April 1. Meanwhile, the EPD report revealed that seven beaches, including Clearwater Bay Second Beach and Discovery Bay, dropped from a good to fair grading last year. A fair grade means fewer than 10 swimmers in 1,000 risk gastro-intestinal and skin complaints. Another three beaches - Yuen Long, Gemini and Kadoorie - declined from fair to poor gradings. This means 11 to 15 swimmers are at risk. Four beaches improved in ranking last year, with Middle Bay and Stanley Main climbing from fair to good and New Cafeteria from poor to fair. Lok Wo Sha improved slightly, going from very poor to poor. The EPD report also raised concern about the increasing incidence of algal blooms at the territory's beaches. Algal blooms can have a red, brown or green pigment and can be triggered by sewage in the water. The red species produces toxins which cause irritations. Mr Broom said algal blooms would not decrease until the Government fully implemented its sewage strategy. This would see waste now disposed untreated into Victoria Harbour collected and disposed through a long sea outfall.