Analisis Efektivitas Saringan Pasir Lambat dengan Media Pasir Silika dan Karbon Aktif dalam Penyisihan Warna, TDS, Amonia dan Nitrit Pada Air Danau Mahoni Universitas Indonesia = Effectiveness Analysis of Slow Sand Filters with Silica Sand and Carbon Active Media in Reducing Color, TDS, Ammonia and Nitrite Concentration in Danau Mahoni University of Indonesia