BIOROCK small onsite wastewater treatment unit installed at mountain huts in Vrhnika, Slovenia
“Biological onsite wastewater treatment plants: using the natural air draft process instead of compressor driven aerated wastewater treatment units”
Owners of buildings that are not connected to the sewerage network have the legal obligation to replace septic tanks with sewage treatment plants by 2017 in Slovenia. Locations in vulnerable water protection areas like Slovenian mountain huts are specially controlled by this sewage treatment legislation. In 2015 the mountain club Vrhnika opted for the sustainable option of biological treatment plants that work without electricity.
The small onsite wastewater treatment unit is installed a few meters from the mountain huts on the mountain above Vrhnika.
“The main reason for the non-electric choice was that the mountain refuge relies on electricity supplied by photovoltaic panels”, says Roman Novak, President of the Mountaineering Society (PD) Vrhnika. The lack of sun during the winter periods reduces the ability of the panels to provide sufficient power for small aerated treatment plants which require electricity. The BIOROCK system was selected due to its innovative water purification bioreactor which is suitable for facilities that are not occupied throughout the year.
For the cottage on the mountain above Vrhnika, which is open only on weekends, a sewage treatment plant with a capacity of up to 5 PE (population equivalent) was chosen. The wastewater from toilets and kitchens was previously diverted to a standard dual-chamber septic tank. Since the hut is located in a second category water protection zones (with a stricter protection regime), water discharge consent had to be gained from the Environmental Agency. This water discharge consent was calculated on the basis of risk analysis for groundwater contamination.
The small onsite wastewater treatment plant (without the addition of chemicals) is comprised of two separate tanks in HDPE, which are interconnected by water pipes and aeration ducts. The raw wastewater initially enters the primary tank (the first stage of the treatment plant, aerobic treatment process) before flowing to the second stage, (the BIOROCK bioreactor), in which the aerobic phase of the process takes place.
The onsite wastewater treatment unit is fully buried in the ground and covered with soil, leaving access to the tanks lids. Only the ventilation cap (air in), bioreactor alarm, and the pre-filter in the primary tank are visible. Two ventilation ducts necessary for supplying the air and creating the natural draft through the system are used for the efficient ventilation of the system. Due to the configuration of the site, the first ventilation duct (bioreactor ventilation) with a diameter of 110 millimeters and positioned four meters high is fixed to a wooden pole, while the second pipe will be the existing drainage pipe from the cottages sanitary equipment running vertically through the roof. Roman Novak says that the site characteristics (grass and fence) will make the above-ground pipes almost invisible.
How does this onsite wastewater treatment unit work in the Mountains, without a power supply? The main difference between non-electric wastewater treatment units and electric aerated treatment units is easy to understand:” Aerated Treatment Units need internal blowers or air diffusers which are essential for supplying the air required for the wastewater treatment process”, explains Zdravko Skubic, director of ARMEX, the supplier of the wastewater treatment plant. In order to aerate the wastewater, electric aerated treatment units use electrical compressors and blowers but with BIOROCK this aeration is provided “with a natural air draft”. As the wastewater treatment plant was built in the mountains the treated water flows out of the pipe network by gravity avoiding the need for any pumps.
"In the first stage of the system, the wastewater is purified by settlement from the top of the bioreactor to the bottom by gravity, with a purification process which is carried out aerobically (using oxygen). The treatment process for the pollutants is as we find in nature, but much faster. The Bioreactor tank is equipped with a multi surfaced material providing a perfect environment for the growth of micro-organisms that degrade the pollutants, (this is material is the BIOROCK media) . These bacteria develop and multiply within a few days in a BIOROCK, whilst the treatment process will need a month or two to be fully operational within an electrically aerated treatment unit.
The challenge of installation works for onsite wastewater treatment units in the mountains
The excavation for the Planina Vrhniko mountain lodge project was a challenge due to the rocky ground. With a length of 140 centimeters between the house and the onsite wastewater treatment unit, the bioreactor dimensions being 115 x 115 x 210 cm, and a soil absorption system the total length of the complete onsite wastewater treatment plant is almost four meters long and more than two meters deep. The bottom of the excavation has been reinforced with concrete, both tanks were placed at the same level; the fall had to be carefully calculated. “The tilt of the wastewater pipes was important”, Roman Novak says. As the purified water from the Soil Absorption System slowly infiltrates into the ground, a perforated pipe system has been placed on a layer of sand with a dimension of 8 to 16 millimeters, and covered with building felt so that the slots do not clog, and then covered with soil.
According to Roman Novak, another advantage of the BIOROCK onsite wastewater treatment unit compared to electric aerated treatment units is the low cost and easy maintenance. According to the load calculations and sludge production forecasts, it appears that sludge will have to be removed every three to four years. After five to seven years the upper layer of the BIOROCK media bags will be rinsed with clear water. Altogether the ongoing maintenance will be minimal, however, the system will require regular commissioning (annual). During this occasion, the pre-filter will be rinsed with clean water and the alarm will be checked (to make sure that the Soil Absorption System is not blocked) as well as the wastewater distribution pipe in the Bioreactor. The natural ventilation system avoids the onsite wastewater treatment unit producing odors in normal operation.
“The ventilation pipes and the water outlet which is 90 centimeters lower for the BIOROCK onsite wastewater treatment unit in comparison with electrical aerated wastewater treatment units are the only small compromises to be accepted when you benefit from a non-electric wastewater treatment plant.” Zdravko Skubic said. “According to his calculations, the cost savings for the maintenance of such onsite wastewater treatment plants is of more than 2,000 euros in 10 years. It is not just about comparing the 40 to 50 euros annual energy savings compared to aerated wastewater treatment units; the BIOROCK system being free from any mechanical parts means also avoiding the risk of changing blowers and compressors which is always necessary with electric aerated treatment units. In addition, the BIOROCK onsite wastewater treatment unit needs less frequent emptying when the primary tank is concerned as the tanks are larger, which is another cost-saving... “