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Treatment of Septic Tank Discharge Using Crushed Glass Filter Media
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Submitted:
18 April 2022
Posted:
19 April 2022
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Abstract
The on-site wastewater treatment system (OWTS) with a sand trench is an economical option for residents in rural areas or the countryside where a centralised sewer system is inaccessible. 2A sand achieves improved filtration, microbial activity and consistent long term performance when compared to gravel or scoria based trench systems. However, it is expensive and only readily available in a few areas across New Zealand. Additionally, it has a reputation for premature blockage when overloaded or compacted. The aim of this study is not only to critically evaluate the performance of Crushed Glass (CG) with respect to 2A sand, but also to investigate different loading rates for CG when treating primary treated effluent from a septic tank. A test rig was designed and constructed to simulate the real environment of the sand column in a discharge control trench. The treatment efficiency of three filters was recorded and compared in this study. Overall, the CG loaded at 25mm/day (CG25) and 50mm/day (CG50) provided an average of 13% and 6% more Total Nitrogen (TN) reduction than the sand filter, respectively. The removal rates for the TN were up to 69.5%. The CG50 filter performed similarly at 50mm/day as the 2A sand filter in terms of Total Suspended Solids (TSS) and Biochemical Oxygen Demand (BOD)5 removal of more than 94%. The CG25 removal rates for TSS and BOD5 at the start of the trial (likely due to residual liquid contamination from the bottles when crushed) improved over the sampling period and ultimately achieved similar results of 92% and 91.3% to the filters loaded at 50mm/day. Lifecycle cost analyses and carbon balances were completed for the two media. It highlighted that the current price of CG is only half of 2A sand, yet it produces significantly less CO2 emissions than 2A sand. A 3-bedroom dwelling could save up to $500 and reduce 200kg of CO2 released to the environment annually when 2A sand is substituted for CG. Based on the findings of this paper, it is likely that the reduced installation cost, lessened environmental impact and theoretical availability will lead to CG systems becoming more common in New Zealand and abroad.
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Subject: Engineering - Civil Engineering
Copyright: This open access article is published under a Creative Commons CC BY 4.0 license, which permit the free download, distribution, and reuse, provided that the author and preprint are cited in any reuse.
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