Membership of Engineers Ireland

Membership of Engineers Ireland

Assignment- Worldwide review of various substrates used in constructed wetlands

Worldwide review of various substrates used in constructed wetlands

Introduction

There are several types of substrates, including soil, sand, gravel, zeolite, limestone, plastic and ceramics etc. It is suggested to collect the substrate materials near the construction location (MEP, 2011). Different region has its own waste removal requirement for particular types of pollutants. Researchers now are trying to find new substrate materials to gain the efficiency to remove different types of pollutants for different kind of CWs. A combination of different substrate materials is also an attempt.

Various Substrates

Soil, sand and gravel are the main three types of substrate materials existed and widely used in constructed wetlands. Recently, many other materials were developed to structure the constructed wetlands as the substrate.

Aiming to reduce the water eutrophication occurring by excessive phosphorus to a particular low level required by EPA of USA, researchers designed vertical up-flow columns with wollastonite as the substrate to test the efficiency of reducing phosphorus in secondary wastewater (Brooks et al., 2000). The research result was excellent. The vertical flow columns with wollastonite showed efficiency to remove soluble phosphorus. Wollastonite showed a promise of being an economic means for the constructed wetland.

For testing different substrate media in the vertical flow constructed wetlands, researchers from Greece reported their research result (Stefanakis & Tsihrintzis, 2009). In the research, Stefanakis and Tsihrintzis designed five small-scale, cylindrical vertical flow constructed wetlands to test the simulated Greece wastewater. The treatment group contained four kinds of substrates, including carbonate rock, igneous rock, zeolite and bauxite. All four materials gained satisfying removal rate of organic matter and nitrogen. But the phosphorus removal rate was lower. Although zeolite and bauxite substrates gained higher nitrogen removal rate and higher phosphorus respectively, the removal rates were not significantly higher than carbonate and igneous rock substrates gained.

Chinese researcher also gained results. Four researchers studied the performance of two substrates made of coke and gravel to reduce Zn, Cu and Pb which are heavy metals (Chen et al., 2009). Although the study was aiming to exam whether the real removal results of these three heavy metals fitted the prediction of the first order dynamic model, these removal results were still able to claim the efficiencies of coke for Zn, Cu and Pb. The Pb removal efficiency is 95-99%. At the same time, the coke substrate constructed wetland only provided an efficiency of 54-91% for both Zn and Cu. It is difficult to claim that coke substrate is more efficient than the gravel substrate from this study.

Wang and Zhang (2012) studied the performance and comparison of bamboo splint and palm silk as the substrate materials for the stable surface flow wetland. The control group is a combination of gravel, cinder, ceramist and sand. The alternative part of Bamboo Splint Group was the additional 10-centimeter-thick bamboo material and 5-centimeter-thick sand filter bed. The alternative part of Palm Silk Group was the additional 10-centimeter-thick palm silk material. The test parameters were COD, TP and TN. The results of removal efficiencies of each kind of pollutants were remarkable.

Removal Rate of Sewage Water with SSFW
	COD	TP	TN
Bamboo Splint	73.97%	61.42%	28.98%
Palm Silk	78.37%	69.42%	24.40%
Control group	66.61%	58.71%	22.23%

Wang & Zhang, 2012

Other materials have also been tested the suitability of applying as the substrate materials. Oyster Shell could significantly adsorb phosphorus in a vertical subsurface flow constructed wetland (Wang et al., 2013). Xin (2013) studied 32 articles and commented on the wood mulch substrate that it was efficient for N removal in a vertical flow constructed wetland. He also agreed that the organic substrate in a HSSF constructed wetland would give a rise of P chemical element (Xin, 2013).

Other than natural materials, industrial materials could also be applied as the substrate. Plastic can be used as the substrate, but the results of removal performance were not better gravel (Burgoon et al, 1989).

Conclusion

Various substrate materials other than soil, sand and gravel were tested and developed. Most of these materials could bring better removal efficiencies than the common materials. The combination of common materials and new substrate materials also showed good pollutant removal performance.

Reference

Ministry of Environmental Protection. (2011). Technical specification of constructed wetlands for wastewater treatment engineering. Ministry of Environmental Protection of the People’s Republic of China. pp3.

Brooks. A, Rozenwald. M, Geohring. L. Lion, L. and Steenhuis, T. (2000). Phosphorus removal by wollastonite: A constructed wetland substrate. Ecological Engineering 15 (2000) 121–132. Available at: http://soilandwater.bee.cornell.edu/publications/BrooksEE00.pdf [Accessed date: May 5^th, 2016]

Stefanakis, A. and Tsihrintzis, V. (2009). Comparison of Various Substrate Media on the Performance of Vertical Flow Constructed Wetlands. The 11^th International Conference on Environmental Science and Technology. Available at: http://www.academia.edu/1095226/Comparison_of_various_substrate_media_on_the_performance_of_Vertical_Flow_Constructed_Wetlands [Accessed date: May 5^th, 2016]

Chen, M. Tang, Y. Li, X. and Yu, Z. (2009). Study on the Heavy Metals Removal Efficiencies of Constructed Wetlands with Different Substrates. J. Water Resource and Protection, 2009, 1, pp: 1-57. Available at: http://file.scirp.org/pdf/JWARP20090100004_45816707.pdf [Accessed date: May 5^th, 2016]

Wang, C. and Zhang, J. (2012). Study on Different Substrates in Stable Surface Flow Wetland. Ecosystem & Ecography. Available at: http://www.omicsonline.org/study-on-different-substrates-in-stable-surface-flow-wetland-2157-7625.1000109.pdf [Accessed date: May 5^th, 2016]

Wang, Z. Dong, J. Liu, L. Zhu, G. Liu, C. (2013). Study of oyster shell as a potential substrate for constructed wetlands. Water Science and Technology: Water Supply. Volume 13, Issue 4, pp 1007-1015.

Xing, A. (2012). Recent Development in wetland technology for wastewater treatment. Halmstad University. Available at: http://www.diva-portal.se/smash/get/diva2:571109/FULLTEXT01.pdf [Accessed date: May 5^th, 2016]

Burgoon, P. Reddy, K. and DeBusk, T. (1989). Domestic wastewater treatment using emergent plants cultured in gravel and plastic substrates. Constructed wetlands for wastewater treatment. Lewis Publishers Inc., pp. 536–541. Available at: https://soils.ifas.ufl.edu/wetlands/publications/PDF-articles/138.Domestic%20wastewater%20treatment%20using.pdf [Accessed date: May 5^th, 2016]

Assignment-Origin of Specific Resistance to Filtration and Relationship between SRF and CST

Origin of Specific Resistance to Filtration and Relationship between SRF and CST

Introduction

The sludge dewatering process could be seen as removing liquid to convert the sludge into solid (Yukseler et al., 2007). In practice, the dewatering process in a wastewater treatment plant would use some special equipment. After dewatering, the sludge become much easier to manage and transport. Because the volume and weight of a sediment cake would be smaller than it was wet sludge. The detailed process of dewatering the sludge sometimes depends on the specific condition of sludge. There are two test parameters named CST and SRF to help to understand the sludge property and dewater-ability.

Definition of SRF and its origin.

The SRF is short for specific resistance to filtration. The SRF test is used for estimating sludge dewater-ability (Sawalha, 2010). SRF is a measure how much the sediment cake resists the liquid being forced out by the dewatering process (Kavanagh, 1980). The theoretical basis firstly appeared in 1933 (Carman, 1933). In 1938, Carman accept Darcy’s law and gave out two assumptions of the cake form and filter media. Then the numerical model appeared to describe the ratio of volume in time that slurry transporting solid onto sediment cake (Smollen, 1986). Then the common used model of SRF was governed (Christensen et al., 1993).

Here list the terms, their meanings and units in the model as below.

Equation Term Table
Term	Meaning	Unit
t	time in the filtration process	s
V	filtrate volume	m3
μ	filtrate viscosity	Pa·s
SRF	average specific resistance to filtration	m kg-1
C	mass of dry cake deposited per unit volume of filtrate	kg·m-3
A	area of the filter medium	m2
P	applied pressure	Pa
Rm	media resistance	m-1

Christensen et al., 1993

In the early period when SRF test was practiced, the SRF results could be influenced by individual differences. To eliminate the phenomena of individual differences happening, the standard test procedure was set. The equipment of the standard SRF test is a Buchner funnel apparatus associated with a vacuum port and paper filter.

Definition of CST and its origin.

The CST is short for capillary suction time. The standard CST test was first developed by Gale and Baskerville in 1968 (Baskerville & Gale, 1968). It also has a standard test process.

Sawalha, 2010. Figure 1.1.1 Diagram of capillary suction time test apparatus

The CST test showed advantages in four aspects, including its short time use, high reliability, easy operation and low cost. There is no special requirement in the CST test. At the meantime, the aims of both CST and SRF are the same. Hence, to estimate SRF value with CST value would bring convenience to the SRF.

Relationship

Generally, when testing on the same sludge, the SRF test result is correlating to the CST result. It has been found that sludge samples showing high CST would show high SRF values at the same time (Baskerville & Gale, 1968). This is more like a qualitative relationship.

Researchers also developed models to investigate the quantitative relationship between SRF and CST. There might be two types of quantitative models to describe the relationship, including the mechanic model and the empirical model.

The mechanic model has been constructed (Lee & Hsu, 1993) as below.

α_av--average specific resistance to filtration (m/kg)

P_cd--capillary suction pressure, assuming a diffusion-like process, in which the paper is unsaturated;

S_o--liquid saturation under the inner cylinder;

A--cross section area (m²);

C_o--solid concentration (kg/m³);

t--time (s);

μ--liquid viscosity (Pa·s);

V--liquid invasion volume.

The empirical model is still being developed by the researchers. There is an equation based on the practical experience as below (Sawalha, 2010).

log_e SRF = 46.128 – 1.346 T + 0.035 T² + 13.760 F/TSS

SRF--the specific resistance to filtration (m/kg);

T--temperature (^oC);

F--the filterability (log_e s/m²)

TSS--the total suspended solids concentration (g/l).

Conclusion

The qualitative relationship between SRF and CST could be found in the early research. This means SRF and CST are correlated. But the quantitative relationship model between SRF and CST is still developing.

Reference

Yukseler, H., Tosun, I., and Yetis, U. (2007). A new approach in assessing slurry filterability. Journal of Membrane Science, 303, 72-79.

Sawalha, O. (2010). Capillary Suction Time (CST) Test: Developments in testing methodology and reliability of results. The University of Edinburgh. Available at: https://www.era.lib.ed.ac.uk/bitstream/handle/1842/4887/Sawalha2011.pdf?sequence=1 [Accessed date: May 4^th, 2016]

Kavanagh, B. (1980). The Dewatering of Activated Sludge: Measurement of Specific Resistance to Filtration and Capillary Suction Time. Wat. Pollut. Control. 388.

Carman, P. (1933). A Study of the Mechanism of Filtration. Part1. Jour. Soc. Chem. Ind. 52:280.

Smollen, M. (1986). Dewaterability of municipal sludges 1: A comparative study of specific resistance to filtration and capillary suction time as dewaterability parameters. Water SA. Vol. 12. No. 3. Available at: http://www.wrc.org.za/Knowledge%20Hub%20Documents/Water%20SA%20Journals/Manuscripts/1986/WaterSA_1986_12_0401.PDF [Accessed date: May 4^th, 2016]

Baskerville, R. and Gale, R. (1968). A simple automatic instrument for determining the filterability of sewage sludges. Water pollution control, 67, 233-241.

Lee, D., and Hsu, Y. (1993). Cake formation in capillary suction apparatus. Industrial and Engineering Chemistry Research, 32, 1180-1185.