KOCHI: Even though the turbidity level in the Periyar caused by the heavy sedimentation from Kallarkutty reservoir is returning to normal in the lower areas, the turbidity in the upstream areas is continuing.
The water at Aluva recorded a turbidity level of 7 nephelometric turbidity units (NTU) on Thursday, according to S. Sabu, Superintending Engineer, Kerala Water Authority (KWA), Central Zone. This was almost near normal as the usual turbidity rate during January-February is 5 NTU, he said.
During the peak of the sedimentation in the river, the turbidity had risen to 79 NTU. Only during heavy monsoon months did the turbidity level rises to such high levels, the KWA authorities said.
All the six pump houses in the upstream area that had to be shut down due to the sedimentation have started functioning.
While the three pump houses under the Executive Engineer, Perumbavoor, are functioning normally, the remaining three further upstream in the Muvattupuzha area are still waging a tough battle against the continuing turbidity.
The KWA officials said they were forced to restart pumping from these pump houses in Kottappady, Keerampara and Kavalangad as these were the only sources of water for four panchayats. Kottappady, Pindimana, Keerampara and Kavalangad panchayats are supplied from Periyar.
Though the pumps had not suffered any damage directly, the sedimentation of silt was itself much damaging, the KWA authorities said. The suction equipment of the pumps, including the foot valve, was completely immersed in the mud. Now, pumping was done after removing a portion of the suction equipment and keeping the pump in an elevated position. More intensive purification process was also being necessitated.
The turbidity level in these areas continued to hover around 30 NTU even now, which was more than the permissible limits. Pumping resumed last week, according to Sadanandan, Executive Engineer, Muvattupuzha.
The retention time of the water inside the chambers had to be increased almost double to allow more settling time. The colloidal turbidity caused by the biogenic silt was more difficult to settle than any normal sediment found in the river water. More quantity of alum and lime were also being required, whereas only a least minimum quantity was usually necessary in the upstream areas of Periyar, which receives clear water that comes from more than one reservoirs.
Both Keerampara and Kavalangad panchayats are located over the Bhoothathankettu dam whereas both Pindimana and Kottappady panchayats are located just below the dam.
The water could turn normal only after a good monsoon season, the KWA authorities said.
Pumping from Ayyampuzha, Manjappra and Kodanad pump houses in Perumbavoor region had already resumed on February 17.