Sludge Treatment Reed Beds
Sludge Treatment Reed Beds (STRBs) are environmental-friendly solution which provide treatment of sewage sludge. Treatment of sludge in a Sludge Treatment Reed Beds is a low energy and low cost solution reducing production of sludge through integrated dewatering and stabilization processes. Research was conducted in WWTP located in Gniewino (Pomerania Region, Poland).
Components installed in the solution
Research was conducted in WWTP located in Gniewino (Pomerania Region, Poland). This WWTP serves 15 000 PE (population equivalent), 35% of wastewater supplied to WWTP comes from diary and food industry and 17% wastewater are delivered from septic tanks.
Total area of Gniewino STRB is 2400 m2 and it consists of 6 beds. Until 2017, the reed bed system was operated continuously. However, due to maintenance challenges, gradual modernisation of some beds was necessary. So far, one reed bed was modernized. It is also planned to build two additional beds to ensure lower load of raw sludge. Reject water (RW) from STRB are recirculated at the beginning of WWTP and added to the influent.
Operational mode
Sludge from wastewater treatment plants creates problems of disposal, which may account for half of the total cost of wastewater treatment. Treatment of sludge in a Sludge Treatment Reed Beds (STRB) is a low energy and low cost solution that uses nature’s own methods to reduce production of sludge through integrated dewatering and stabilization processes.
STRBs are typically built as ground objects with horizontal layers of gravel and sand mostly planted with reed. It is very important to build drainage system connected to the ventilation chimneys. This prevents the formation of anaerobic conditions and ensures properly conditions for dewatering and stabilization of sludge. During the operation of STRBs there are three periods (commissioning, full operation and emptying). During the operation of the first commissioning period (which lasts about 2 years), dose of dry matter of sludge should be lower than designed values. During operating reed systems are periodically irrigated with sewage sludge with a low dry matter content (from 0.5% to 1.5%). Resting periods (breaks in sludge supplying) may take several days, depending on weather conditions, the age of a system, the dry matter content in sludge as well as thickness of accumulated sludge. Before emptying bed should not be used for some time (preferably in summer) to increase the content of dry matter and also improve sanitary properties.
The method of STRBs is predicted for long-term management of sludge, which is about 10-15 years. Raw sludge is directly supplied into special bed planted with reed. An important element of the reed system is the correct construction of the drainage layer, which drains leachate and ensuring the inflow of air. Due to presence of reed, the efficiency of sewage sludge dewatering is very high, and content of dry matter reaching up to 40%. In STRBs sewage sludge is not only dewatered but also stabilized. After this process, sludge is characterized by a chemical composition similar to that of humus. Additionally, obtained product is safe as microbiological regards. STRBs do not require the use of additional chemicals to improve the process. This is a low-emission and low-energy-consuming method, because electricity is needed only to pumps supplying sludge to the system. STRBs are especially useful in rural areas and housing estates where economic considerations limit the use of expensive mechanical equipment. These systems can be established in any area and are simple to build and operate.
Results
1. Sewage sludge quality
The samples of sludge from STRB were collected three weeks after supplying on bed. In analyzed bed sewage sludge has been stored for three years. The oldest layers of sludge were placed at the bottom of bed, while the newest layers of sludge were on the surface.
Quality of raw and treated in STRB sewage sludge from WWTP in Gniewino:
Based on obtained results it was found that time of dewatering has a significant influence on dry matter content. After three weeks content of dry matter increased from about 1% to almost 7%. The increase caused significant reduction of sludge volume. Longer time of dewatering resulted in further increase of dry matter content. Research shows that also in surface layer the increase of organic matter in comparison to raw sludge occurred. However, in bottom layer treated for longer time the organic matter content significantly decreases. The sludge from STRB in Gniewino has very high concentration of nitrogen and phosphorus. The increase of dry matter content and at the same time the decrease of organic matter content (especially in bottom layer) showed that sewage sludge treated in STRBs was dewatered and stabilized at the same time. Moreover, high concentrations of nitrogen and phosphorous and presence of potassium provide good preconditions for potential use of sludge as fertilizer.
Average concentration of chromium, copper, nickel, zinc, cadmium and lead in raw and treated sewage sludge from WWTP in Gniewino as well as the permissible values of metals for the agricultural use according to The Regulation of the Environment Minister (2015) in Poland:
Based on obtained results it was indicated that selected heavy metals concentrations in sewage sludge from STRB in Gniewino were significantly below the permissible values for the agricultural use.
2. Reject water quality
In case of reject water from STRB in Gniewino low value of BOD (from 40 to 45 mgO2/dm3) was found. However, values of COD were much higher and they amounted to 186-221 mgO2/dm3. Total nitrogen concentration was from 194 to 247 mg N-NO3-/dm3 and ammonia nitrogen from 129 to 166 mg/dm3. While nitrate nitrogen was from 24 to 118 mg/dm3 and was lower in comparison to values from literature. The concentration of orthophosphates was from 96 to 100 mg PO43-/dm3. The concentration of chlorides was from 637 to 697 mg/dm3 and it was higher than they concentration in effluent from WWTP, which as average was 560 mg/dm3. Concentration of sulfates was from 64 to 79 mg/dm3 and it was below the value of sulfates in effluent, which as average was 119 mg/dm3.
The quality of reject water and it changes in time:
