Beach wrack treatment in reed beds
Reed bed systems for many years have been used for treatment of different kinds of sewage sludge from WWTP. This solution is an environmentally friendly technology, which could be used also in case of beach wrack- specific material which shows properties similar to sewage sludge.
Main processes occurring in deposit are stabilization and dewatering, which are based on mechanisms of natural wetlands, without use of additional chemicals. This method also has low emissions and low energy consumption, which adds an additional advantage. In the core of a well-working deposit are correct design and reliable project realization. Reed bed systems are characterized by their simple construction. An important element of this technology is drainage, which allows to outflow reject water as well as to ensure the constant air supply. Reed presence provide appropriate conditions for the stabilization process and also contributes to dewatering. Preliminary effects of using the reed bed system in case of beach wrack are promising.
Research is conducted in WWTP in Swarzewo (Pomerania Region, Poland). This place was chosen due to small distance (about 13 km) from the collection point of beach wrack material- Rzucewo’s beach (Pomerania Region, Poland). Rzucewo is characterized by significant accumulation of macrophytes among other Baltic beaches. This may be related to the location on the cost of Puck Bay, whose shallow waters are recognized the most productive area on the Polish coast.
Localization of pilot plant of RBS (Swarzewo) and collecting point of beach wrack (Rzucewo)
The Puck Bay has specific conditions: large inflow of freshwaters from rivers and groundwaters, low salinity, variable temperature ranges from 0°C to 25°C. Beaches located on Puck Bay’s coast have fine to medium sand.
In autumn 2019, the pilot facility of reed bed system was built in the WWTP in Swarzewo. It consists of two cubes (dimensions about 1m x 1m x 1m) with felled tops. Each object is divided into four quarters using plexiglass walls. Three horizontal layers of filtration material create bed filling:
- sand with grain size 0,32 – 2,5 mm constitute top layer (growth layer) with thickness about 15cm;
- gravel with grain size 2,5 – 10 mm constitute intermediate layer with thickness about 20 cm;
- aggregate with grain size 10 – 40 mm constitute the deepest layer (drainage layer) with thickness about 25 cm.
Above the deposit filling there is free space for discharge material- beach wrack for testing. Feeding material into cubic quarters is done manually.
Four seedlings of Phragmites australis are planted in the growth layer of cubes quarters. The deepest layer has drainage made separately for each of eight parts of deposits, which is connected with ventilation chimneys mounted above designed high of drainage. It allows to collect reject water and provide aeration and ventilation conditions in tanks needed for processes occurring in beds. During dewatering, most of the water is used for reed needs, as well as in the evapotranspiration process, and also outflows as reject water, which stays in a collecting, flexible pipe from each quarter until the collection for testing.
Reed bed system for beach wrack is an innovative solution, so it requires testing of all aspects in order to make assessment of this technology and provide recommendations on how to use it in case of beach wrack.
The pilot of a reed bed system was built in the autumn (end of October) 2019 and it is in testing phase of the potential application in case of treating beach wrack.
Beach wrack quality
Charging beach wrack material for deposits takes place on every month. Each time the amounts and proportions of delivered material were the same for individual parts. So far, there have been four deliveries for four quarters:
- Quarter ”B”- 10l of beach wrack;
- Quarter ”C”- mix of 10l of compost with 10l of beach wrack;
- Quarter ”D”- 15l of beach wrack;
- Quarter ”F”- mix of 5l of compost with 5l of beach wrack;
and one from August supplying additionally two quarters:
- Quarter "G" - 5l of shredded algae;
- Quarter "H" - 5l of shredded algae mixed with compost.
The samples of beach wrack or beach wrack mixed with compost are collected about 4 weeks after supplying on deposit, before the next delivery.
The changes of quality of beach wrack in reed bed system is Swarzewo
The obtained results show that dry matter content depends on the dewatering time. After one month of material being in deposit, an increase in dry matter content from 5% to 62% can be observed. The decrease in water content is associated with a decrease in volume and mass of material in bed. At the same time, organic substance content decreases from 50% to 22%, and mineral substance content increases from 50% to 78%, which shows that the material is also stabilized. The relatively high content of phosphorus shows that the treated material beach wrack can be a potential fertilizer.
Reject water quality
In case of reject water from pilot object of reed bed system in Swarzewo value of pH was from 7,85 to 8,06 and organic matter concentrations expressed in COD was from 161 to 816 mg O2/l. In comparison to seawater, COD of reject water was higher. Total nitrogen concentration also took higher values in reject water than in seawater and it was from 40,10 to 91,40 mg TN/l and ammonia nitrogen from 0,07 to 0,57 mg NH4/l. While nitrate nitrogen was from 12,60 to 54,80 mg NO3/l and nitrite nitrogen was from 0,08 to 0,51 mg NO2/l. Total phosphorus was from 0,36 to 0,89 mg TP/l and the concentration of orthophosphates was from 0,25 to 0,46 mg PO4/l.
The quality of reject water and it changes after supply of fresh material to deposit
So far, the main method of utilization of beach wrack was storing it on a landfill, and the general price of the service consisted of several costs:
- storage of material on a landfill,
- transport of the material from the collection point to the landfill,
- work of people collecting material on the beach and working on the landfill.
This method does not bring any benefits and beach wrack was treated only as a nuisance that needs to be eliminated. The use of RBS for beach wrack processing is an environmentally friendly alternative that can additionally bring income from selling produced fertilizer. Additionally, RBS is a cheap solution to build and operate. The reed system does not use any additional substances supporting the processes taking place in the deposit and also it uses energy only for operation of pumps delivering material to the deposit. Moreover, this solution easily fits the surrounding landscape, so it could be built near a beach, which could allow decreased costs of transporting the material.
Ultimately, it can be assumed that costs associated with both methods are similar. However, use of RBS gives possibility of additional income from the sale of fertilizer and allows of changing nuisance into resource, which definitely gives this method an advantage.