Authentic and Reliable Suspended Solids Measurement Processes for Stormwater

http://www.envicenter.com/iwa2008/web/index.php?option=com_frontpage&Itemid=1

 

論文發表人: 詹立成 (加州大學洛杉磯分校土木及環境工程研究所博士班)

 

總懸浮固體需樣品混合均勻，前期研究低速下易造成實驗誤差，攪拌強度取決於攪拌過程，如人工，機器或其他拌勻方式，及以人工或試管分次樣品，機器自動分次樣品，未定義方式易造成誤差。本研究分析實驗偏差值並且建立最好實驗過程。

實驗依據水及廢水處理標準程序執行，以微矽珠與沉積土為實驗物質，方式有搖勻或拌勻後人工及試管分次樣品，加入緩衝板或方形容器後人工及試管分次樣品，及機器拌勻試管分次樣品。

搖勻或拌勻試管分次樣品較人工分次樣品準確，由於人工分次樣品較難以得到準確容量。拌勻較搖勻準確。以試管分次樣品及250RPM攪拌速度，機器拌勻於直徑小於90毫米顆粒獲得74%回復率，相較直徑大於125毫米顆粒只得8%回復率，歸因較大顆粒沉積於底部。增加攪拌速率會導致渦流現象產生而攪拌不完全，加入緩衝檔板或方形容器可減少渦流現象，但顆粒卻於燒杯角落沉積。人工傾倒較難反應實際濃度，搖勻無法使顆粒分布均勻，導致次樣品濃度不一。加入緩衝檔板或方形容器並不適用於攪拌。在700RPM攪拌速度，小於250毫米懸浮微粒可得到超過95%回復率，大於250毫米可得到72.4%回復率。標準燒杯使用700RPM以上攪拌速度的拌勻方式，與使用試管分次樣品可得到最準確回復率。

 

The total suspended solids (TSS) standard method requires well-mixed samples for analysis. The previous work has shown that TSS results are unreliable below certain mixing speed. The required mixing intensity depends heavily on the mixing processes. The mixing method can be shaking, stirring, or even not specified. Transferring subsamples can be pouring, pipetting, or automatically distributing by using the churn splitter. The various mixing methods and the lack of a well defined procedure can cause errors in recover. This study analyzes the deviations among mixing processes and establishes the best mixing process for the TSS experiment.

The TSS standard method was performed based on Standard Methods for Examination of Water and Wastewater. Silicon beads and embedded sediments were used in the experiments. Several mixing processes were performed. Pour and pipette subsamples after shaking or after stirring. Pipette subsamples after stirring in the container with baffles or the square shape of the container. Pipette after stirring by the gang-mixer.

Pouring subsamples is less precise than pipetting either after shaking or stirring. It is also more difficult to obtain accurate volumes when pouring. Stirring yields more precise results than shaking. With pipetting subsamples and 250 RPM mixing speed, experiments using gang-mixer show 74% recovery rate of the particles smaller than 90 mm but only 8% recovery rate of the particles larger than 125 mm. Larger particles can be observed at the bottom of the sample collection beaker. Simply increasing mixing speed produces a large vortex when prevents mixing. Beakers with baffles and square container were utilized to eliminate the vortex but resulted in dead zones that trapped suspended solids. The experiments show that pouring has poor recovery rate of the real concentration. Shaking samples may result in partial mixing and uncertain particle concentration of the subsamples. Beakers with baffles and square containers are unsuitable for the mixing. Particles smaller than 250 mm have more than 95% recovery rate and 72.4% recovery rate for those larger than 250 mm at 700 RPM mixing speed. In sum, using stirrers, stirring at the mixing speed of 700 RPM or higher in a regular beaker, and using pipettes transferring subsamples gives the best recovery rate.