Biochar based filtration structure in Otsolahti
Otsolahti is located in Tapiola, Espoo. It is a shallow, almost closed sea bay, which has become shallower and increasingly eutrophicated over the years. In the past, storm water run directly through the storm drain into Otsolahti. As part of the project “Kaupunkivesistöt kuntoon” (2017-2019), financed by the Ministry of Environment of Finland, a biochar filtration structure was built in Otsolahti park, next to the bay in order to protect the bay from eutrophication.
The storm water drain network collects stormwater from a large area in Tapiola and the busy main highway arteries run through the area. The filtration structure gathers storm water from an area of 16 ha. In the filtration structure, stormwater runs from a large storm drain embedded under the park into an underground structure which infiltrates solid matter, nutrients, heavy metals and oil from the water before the water runs into the bay. Besides environmental protection, another aim of the solution was to verify and gather experience of usage of biochar based storm water filter structure in a city, such as investment and usage costs, environmental effects, e.g. nutrient recycling, operating life, a need for maintenance and possibilities for reusage of biochar.
Components installed in the solution
The filtration structure of the Otsolahti is implemented entirely underground. Only visible parts of the structure are the manhole covers. In the filtration structure, storm water runs from a large storm drain embedded under the park into an underground filtration structure. First, a sand trap separates sand and solid matter from the storm water, which prevents the blocking of the filtration structure. From the sand trap, the water is evenly distributed in the filtration structure by means of storm water cassettes. There are 140 cassettes (600 mm x 600 mm x 1200 mm), total capacity 57.5 m3. Storm water cassettes provide additional volume for temporary storage and retention of water. The filtration layer cleans the water as it filters through sand and biochar. After the sand and biochar filtration, the water is collected with underdrain carpet and pipe into the storm water drain.
The storm water filtration system in Otsolahti of Tapiola is based on the ability of biochar to retain water, nutrients and contaminants. Due to its porosity, durability and its large specific surface area, biochar is well suited for treating storm water. Biochar is produced by burning biomass, usually wood, at a very high temperature without oxygen. In this form, coal is very persistent and long-lasting, so the use of biochar in various structures also helps to curb climate change.
Before the installation of the filtration structure, storm water flow in the stromwater drain was measured in order to find out the volumes and intervals of the incoming water. According to the results the flow 29 November 2017-3 January 2018 was at highest 270 l/s and on average 26.5 l/s. Rainfall within the same period was 137.6 mm. In the spring time 3 April - 3 May 2018 the flow was max 88.5 l/s and on average 2.3 l/s. Rainfall was 27 mm.
Based on discrete sampling, more than half of suspended solids of the incoming storm water was retained already in the sand trap. In total, 89 % of the suspended solids was retained by the structure. Also turbidity decreased by 83 % during the sampling period.
Based on the discrete sampling results, the filtration structure does not have an effect on electrical conductivity. However, there was some variation in the electrical conductivity of the incoming water (393-989 μS/cm) during the continuous measuring while the electrical conductivity of the outcoming water was stabilized between 499-613 μS/cm.
TOC and DOC were not affected by the filtration.
Phosphorus was effectively removed from storm water. Average reduction of total phosphorus in the discrete samples was 70 % and for dissolved phosphate phosphorus 68 %. Based on the results phosphorus was removed especially in the biochar filtration and not so much in the sand trap.
Nitrogen removal was not as efficient. There was great temporal variation in the concentration of nitrogen in the incoming water especially between spring and autumn sampling periods. In spring, total nitrogen concentration was lower than in autumn. At the same time the concentration of nitrogen in the outcoming water rose and almost doubled compared to the incoming water. Almost all the nitrogen was nitrate. It is possible that some outside source of nitrogen, such as fertilizers. At the same time also sulfate concentration rose and was higher in the outcoming water than in the incoming water.
The ability of the filtration structure to retain metals varied. On average sand trap retained all metals and concentrations of mercury, cobalt, chromium, nickel, lead, zinc and vanadium decreased by half. More cadmium, copper, lead, zinc and vanadium was retained in the filtration. Concentrations of the rest of the analyzed metals even increased in the outcoming water. The biochar filtration did not retained dissolved metals effectively. Only concentrations of dissolved copper and zinc decreased.
Majority of polyaromatic hydrocarbons (PAH) and oil hydrocarbons were retained already in the sand trap, 63-83 %. In total 74-88 % was retained by the filtration structure.
According to this experience the need for maintenance and costs related to it increase remarkably, if separation of sand and other solid matter is not effectively handled already before stormwater gets into the filtration structure. If the structure clogs easily there is a need for maintenance every year while well functioning sand separation decreases the need for maintenance interval to 5-10 years and it is enough that the sand traps are emptied regularly.
In order to avoid clogging it is important to select the materials in the filtration structure carefully. For example quality of the water and coarseness of the filter cloth have an impact on how easy the structure clogs. It is better to choose coarse filtration material even on the expense of retention time. Cost effectiveness is reached through long maintenance intervals even tough filtration results weakens due to short retention. Constant flow of water decreases the risk of clogging.