求翻译：取80mL污泥浓度为4g/L的污泥样品于烧杯中，在冰浴中超声（50W）振荡15s。取10mL于1200r/min离心2min，上清液在650nm处测吸光度A；剩余污泥置于磁力搅拌器上低速搅拌10min，静止15min后，在650nm处测上清液吸光度B。是什么意思？

4g/L take 80mL sludge concentration in sludge samples in the beaker, ultrasound in the ice bath (50W) oscillation 15s. Centrifugal 2min take 10mL to 1200r/min, the supernatant measuring absorbance at 650nm a; low speed mixing 10min on excess sludge on the magnetic stirrer, still after 15min, supernatant absorbance measurement b at 650nm.