In case of very high level the overflow device might start and discharge the sewage directly into the sewer system.
This might happen because of excessive inflow or a shredder-pump malfunction due to an obstruction.
In the event of a very high level, the second shredder-pump group automatically starts, while an alarm signal is sent with a request for service.
In the event of a very low level, the shredder-transfer pump group might operate in an empty vessel. A first warning signal is sent and if the situation is not corrected an alarm signal is sent with a request for service.
The probes, in parallel with the continuous analogic probe, control sewage level changes between attachment and detachment points of the shredder-transfer pump group.
The level system is designed specifically so that the transfer pump starts 30 seconds after the shredder to ensure proper stirring in the shredding vessel.
The sewage containing shredded solids and dispersed material is pumped to the separator/compactor. The separator/compactor consists of a finely drilled plate for sewage filtering and rubber rollers on a drilled plate for compression and dehydration of the separated solids.
The separator/compactor starts at the same time with the shredder in order to remove possible residual fouling from the previous cycle.
Gross separated and dehydrated solids are removed by spinning brushes and discharged into a collection bag, while the filtered sewage is transferred into the disinfection basin by centrifugal pumps.
The pump start and stop commands are controlled by the level probe, while the continuous analogic probe continues operating in parallel.
When the shredder/pump cycle is finished the compactor continues operating for a few minutes while the counter-wash and drilled-plate-wash systems start.
An electromagnetic sensor allows for monitoring sewage flow in the disinfection vessel and controls the disinfectant - 15% peracetic acid - dosing pump.
The disinfection vessel is designed with a usable volume to guarantee an average 30-minute contact time and is equipped with a stirrer to ensure constant stirring of the sewage.
Disinfectant dosing is also controlled by a sensor monitoring the dissolved oxygen concentration to avoid unnecessary overdosing.
The dosing pump controlled by the pH sensor maintains sewage pH between 7 and 8.
The disinfected water is discharged into the sewer system by gravity from the upper part of the disinfection basin, which is equipped with a bottom discharge hole to allow for periodic cleaning.
The shredding vessel and the separtor/compactor are equipped with overflow tubes connected to the sewer system designed to prevent spill over the floor where the equipment is installed.
Dimensioning
To guarantee the highest operational standards, all Newster SWT hydraulic equipment is dimensioned to operate at 10 m3/h continuous flow rate with up to 50% continuous overload.
The system starts automatically when sewage starts flowing in the catch basin from the hospital sewer system and continues operating until sewage keeps on flowing.
The shredding vessel also provides an intermediate basin to collect an adequate amount of sewage and avoid the system starting and stopping continuously when flow rate is low.
The separator/compactor is dimensioned to operate at 20 m3/h flow rate with sewage containing up to 5% of solids in suspension.
Disinfectant
The disinfectant of choice is 15% peracetic acid, a widely used agent in sewage treatment.
For best results, treated sewage pH should be mainteained between 7 and 8.
This is usually achieved thanks to a pH-sensor controlled system dosing a 15% lye solution into the water flow.
The peracetic acid dosing system is controlled by a probe measuring the amount of oxygen dissolved, which is usually maintained at a proper level by dosing 5 ppm of peracetic acid into the sewage.
Outflowing water quality and quantity
Treatment effectiveness has been tested both on hospital sewage and common sewage containing high concentration of organic and fecal substances.
In the second case raw sewage was used containing solids dispersed and filtered by the system.
The purpose of these tests was to compare the concentration, before and after treatment, of oxygen dissloved, escherichia coli, total coliforms, bacterial charge at 37°C and COD. This latest value was taken to evaluate the effect of chemicals.
After installation, regular service is performed to ensure equipment efficiency and treatment effectiveness.
