Microstrainer for removal of algae in drinking water treatment

Abstract

The algal management procedure, which is most actively investigated, is based on the reduction of nutrient loading which reduces algal bio mass. This is a difficult task as it requires a consensus of the community. Thus, some physical, chemical and biological control methods are being practiced to control algae. Among the available algal control methods, microstrainer is one of the cost effective physical control methods which has been developed to remove algae before water passes through the conventional water treatment processes. Microstrainer consists of a finely woven stainless wire cloth (25-35ìm) mounted on a revolving drum which has a rotation speed of 315 rpm during the operation. Very fine suspended matter such as algae and plankton can be trapped on the screen and washed away by highpressure water jets for disposal.

In Sri Lanka, currently, Only one microstrainer is being used in Polonnaruwa water treatment plant to remove algae from raw water before the treatment process. Present study was carried out to study removal rate of algae through microstrainer as no studies have been done so far in this regard. Two field experiments were carried out during June and October period based on literature of phytoplankton species composition variation of the Parakrama Samudra reservoir where water is collected to the treatment plant. Water samples were collected from different zones namely raw water where before the microstrainer, immediately after microstrainer, algae washed water from the microstrainer and filtered water. 100 ml of portion of samples were fixed with acidified Lugol’s solution at final concentration of 1% by natural sedimentation and enumeration was done under the microscopy.

Microsystis aeruginosa was the dominant phytoplankton while Pediastrum spand Nodularia sp were co-dominates during June and October respectively. Removal rate of M.aeruginosa, through the microstrainer was 94% in June and 85% in October sampling time. In June Pediastrum duplex, Pediastrum simplex,and Scenedesmus sp removal rates were 88%, 82%, and 87% respectively and in October Nodularia sp, P.duplex, P.simplex (meyen) and Scenedesmus sp were 70%,99%,58% and 99% respectively. Merismopodia sp, Pseudotetraedron spand Peridinium sp were recorded in water collected immediately after microstrainer and filtered water. Analysis of algae washed water samples revealed more than 90% of algae in raw water removed through the microstrainer.Thus, present study showed that the microstrainer is an efficient, cost effective and environment friendly method to remove algaem from raw water before conventional treatment processes. This may effectively reduce the cost for back wash processes and treatments of algae during the treatment processes. Therefor further studies regarding, operational efficiency and cost effectiveness should be carried out to ascertain the potential use of microstrainer for removal of algae in water treatment processes as providing safe drinking water is a global challenge in the near future due to production of algal biomass in reservoirs.