Increasing sludge disposal costs in Sweden are the incentive to search for a new solution for septic sludge treatment. Commonly septic sludge is hauled to large central wastewater treatment plants and processed together with their sewage sludge. The restrictions for applying sludge on land are so stringent that most central plants can not meet the requirements and have to use more expensive ways of sludge disposal, e.g. drying and incineration. Septic sludge from households, however, is far less contaminated and may usually be applied on land if it is treated separately.
The city of Östersund with a population of about 50.000 is located in northern Sweden. The sewer authority wanted to treat their septic sludge separately. The total sludge volume is around 6000 m³ per year whereby most of the sludge is hauled from April to November because it would be very difficult to empty septic tanks during harsh winters in northern Sweden. The authority required removal of debris, grit and gravel from the sludge as well as its dewatering. Last, but not least, they wanted to have a very reliable system that is easy to operate.
In 2004 the sewer authority hired consultants to find a solution for separate septic sludge treatment, and they contacted our Swedish subsidiary, HUBER HYDROPRESS. We could offer our SeptageTreat® Solution. The system was installed during spring 2005 and commissioned shortly after.
Collected septic sludge is hauled in tanker trucks to the central wastewater treatment plant of Östersund where a separate septic sludge treatment system was installed. The liquid sludge is pressed through our septage receiving station Ro 3.3 that includes a Ro 2 wedge wire screen with 4 mm spacing and a screen basket diameter of 1.2 m. It is also provided with an integrated IRGA for washing, dewatering and compaction of the screenings. The Ro 3.3 unit also includes a grit trap and grit classifier. The pre-treated sludge is discharged into a storage tank with a volume of 200 m³.
From the storage tank the sludge is pumped to our screw press RoS 3 size 2, but first it is blended with polymer that is prepared in a polymer station. Flocculation occurs in a flocculation reactor wherefrom the sludge overflows into the screw press. The inclined screw press discharges dewatered sludge cake through a chute at its upper end.
The thin sludge has a concentration of only around 1 %DS, which is rather low. The sludge cake’s solid concentration is continuously above 30 %DS. The operators even found out that they could achieve 37 %DS when they increased the pressure within the screw press by adjusting its cone at the discharge end. Normal flow to the screw press is between 7 and 8 m³/h. Our screw press operates fully automatic after it is switched on. It shuts down and washes itself when the level sensor in the buffer tank provides a low signal. Operator attention is minimal. The operators are also very pleased with the comparatively low polymer consumption. Filtrate of good quality is discharged into the wastewater treatment plant.
By Ulf Bengtsson, General Manager of HUBER HYDROPRESS, Sweden