It has been estimated that in 2006 there was sufficient capacity to treat 52% of municipal residential wastewater.[7] By June 2010, there were 1,519 municipal wastewater treatment plants in China and 18 plants were added each week.[15] Many existing plants are being expanded and upgraded to include a tertiary treatment stage for nutrient removal to comply with more stringent discharge standards introduced in 2002. In terms of technologies, in 2007, the most commonly used technologies were various forms of activated sludge, including oxidation ditches (24% of all plants), sequencing batch reactors (11%), conventional activated sludge (5%) and membrane bioreactors. In small communities, plants using only primary treatment were still common (15% of all plants).[16] An analysis in 2004 by the Ministry of Construction showed that, according to treatment plant records, most plants operated in compliance with discharge standards.[17]