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GRACE ROBINSON HYDEChief Engineer and General Manager
Puente Hills LandfillGas Trench Video Survey Report
Prepared For:
Solid Waste Landfill Operations - Puente Hills Landfill
Prepared By:
Jason Kim, Student InternSolid Waste Research/Monitoring
Date: August 30, 2016
TABLE OF CONTENTSSection Page
1. Introduction
This report presents the results of a gas trench video survey at the Puente Hills Landfill. The survey involved the video inspection of 67 pre-selected horizontal landfill gas collection trenches for overall integrity.
1.1 Objective and Approach
The objectives of this video survey are as follows:
Assess the quantitative feasibility of the recovery of a methane flow at the landfill horizontal gas collection system.
To quantify the potential inundated trenches and the damaged trenches. To provide PHLF with a tool to guide potential project development in making informed
decisions regarding additional investigations on the recovery of a methane flow.
The approach taken for this video survey has included the following tasks:
Reviewing the trench conditions and available background information, including configuration, gas quality, gas flow, and its location.
Visiting the site to observe trench integrity and operations and meeting with the landfill technicians.
Estimating the project methane flow by totaling flow rate and calculating the average methane flow from the selected parameters.
1.2 Landfill Gas Background
A solid waste landfill can be viewed as a biochemical reactor, with solid waste as the major input and landfill gas as the principal output. Landfill gas is the product of the decomposition of organics in municipal solid waste. In general, this decomposition process results in the generation of methane, carbon dioxide, and trace gases. Both of the two primary constituents of LFG are considered to be greenhouse gases which contribute to global warming. Methane present in the raw LFG is much more potent greenhouse gas than carbon dioxide. Therefore, the capture and combustion of methane in an LFG flare or an engine generator, results in a substantial net reduction of methane emissions.
There are two natural pathways by which LFG can leave a landfill: by migration into the adjacent subsurface and by venting through the landfill cover system. Without capture and control the LFG will eventually reach the atmosphere. While the methane emission rate will decrease after a landfill is inactive, a landfill will typically continue to emit methane for many years after its closure.
A common means for controlling LFG emissions is to install a LFG collection and control system. The landfill gas is captured by a gas extraction system and utilized for energy recovery. Capturing the landfill gas also reduces odors and gas emissions to the atmosphere.
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The Puente Hills Landfill began operation in 1958 and closed on October 31, 2013. The Puente Hills Landfill has an extensive landfill gas collection system as seen in Figure 9. All collected landfill gas is used for energy recovery at the on-site power plant. However, since its closure, a more than anticipated decline in landfill gas production has been observed. In recent years, the production of landfill gas has been deteriorating at a faster rate than expected. Among other things, deteriorating and flooding landfill gas horizontal trenches were identified as a possible source of reduced collection. In an effort to identify the cause of the decrease in landfill gas collection from specific extraction trenches, a video survey of various locations was conducted.
2. Equipment
The Ridgid SeeSnake Pipe Inspection System was used to diagnose and locate problems in the trench conditions. It consists of a durable reel and camera system along with a camera control unit. The system measures distance as the camera is pushed into the pipe. The camera cable or “pushrod” can extend up to 200’ into the trench. The pushrod is flexible yet stiff enough to push the camera into the pipe as seen in Figure 1. In addition, in case of the camera head encountering obstruction which may lead to premature failure of recording, attachments are used to evaluate the camera head allowing it to move pass small obstructions and to remain centered in the pipe or lateral, as seen in Figure 2 and Figure 3. The attachments also prevent sediment and mud from accumulating on the camera lens which interfere with the recording. The camera control unit can display the following parameters on the image: time, date, units of measure, & trench section. All inspection information is stored on a USB flash drive for future evaluation.
3. Method
Prior to conducting the video inspections, historical graphs of landfill gas flow and gas quality were reviewed in order to identify video survey candidate trenches. In particular, the primary selection criteria utilized were the fluctuation in landfill gas quality and gas collection in the trench. Trenches that showed significant decreases in gas quality and flow over a relatively short time period were a higher priority. Based on the graph analysis, a total of 110 gas trenches were identified as candidates to be surveyed and 67 trenches have been inspected. The video surveys display the current physically condition of the trench interior collection pipes. The survey results will serve as a guideline to aid in the rehabilitation of dysfunctional landfill gas trenches.
Prior to the start of the trench video survey, the piping is inspected for excess flooding or possible liquid surging. This is done by simply loosening the bolts on the trench lateral and observing if a large amount of liquid spills out. Also, the piping inspection is to avoid an uncontrollable spill that can result in regulatory reporting inconveniences. If excess liquid build-up is found, a vacuum pump truck is used to remove the liquid. The video survey can only be conducted on the trenches deemed safe to open, as seen in Figure 4.
Once the trench is confirmed safe to survey, the system vacuum is adjusted to prevent excessive air intrusion during the survey, as seen in Figure 5. If the control valve on the trench is fully open applying
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high vacuum, the valve is adjusted down in order to reduce vacuum and air intrusion. The SeeSnake system is turned on and the interface displays the correct parameters. Then the camera head is inserted into the trench lateral to start the video inspection, as seen in Figure 6. As a backup to the digital inspection, the trench integrity is also documented on the paper.
4. Results
From June 2016 to August 2016, sixty-seven trenches have been surveyed. Thirty-five of the surveyed trenches were flooded with liquid, thirteen were damaged or crushed, four were both deformed and flooded, and nineteen were in good physical condition, as shown in Figure 7 and Figure 8.
Flooded trenches show characteristics of poor landfill gas collection since the liquid impedes the collection of landfill gas. Deformation in the trench piping caused by differential settlement also contributed to the ongoing deterioration of gas collection. There were several types of deformation identified in the trench lateral: dislocation of the lateral caused by the tension from both ends of the pipe; shearing causing the pipe or trench to laterally separate; and crushing caused by excessive overburden stress from the refuse. Based on these findings, a recommendation for each trench case was offered. The most meaningful action is to install pumps to remove liquid and restore landfill gas collection. A collapsed or deformed trench can no longer function to convey landfill gas. The trench inspection progress for summer is summarized in Figure 10.
5. Analysis
5.1 Introduction
For projecting methane recovery in association with a pump installation on the Puente Hills Landfill gas collection system, the method of monthly mass balance on dewatering trench candidates will be applied. The conservation of mass allows monthly methane flow rate to be added as a group. This addition of methane flow is utilized to assess the impact of pumping flooded trenches. The data analysis on behaviors of methane flow will be evaluated to project future recovery and improvement of methane flow on trenches that need to be dewatered. The specific projecting approach is discussed below.
5.2 Landfill Gas Recovery/Improvement Modeling
Following parameters from dewatering trench candidates are considered for analysis: date of pump installation and section number. The section number and the pump installation date may have correlation in terms of configuration and the life span of the trench lateral. These parameters will be vital benchmarks to group trenches for an accurate estimation of methane flow. Particularly, the date of pump installation indicates how long the trench has been dewatered and it also displays the performance of LFG generation afterward. Hence, a utilization of parameters will be a vital indicator to make a decision on whether the flooded trenches should have a pump or vice versa. In addition, the date of trench installation is an essential factor to this analysis. The location and the material are analogous to the behavior of LFG. However, the trench installation date was not implemented due to the insufficient number of trench candidates to make correlation.
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For a mass balance analysis, the trenches with pumps installed in 2016 were considered only. From there, the trench section was examined for further correlation. If trenches had less than two of the same section, those trenches were excluded to be grouped by section numbers.
Based on the video trench survey and a review of the trench lateral configuration, Figure 11 had a pump installed in July 15, 2016. After one month of pumping liquid, the observation evinced an increase of methane flow by 15 CFM and the flow rate by 21 CFM. This recovery is the manifest of effectiveness of pump removing the condensates inside the trench lateral. To consolidate the healthy recovery of methane flow by a pump, continuing monitoring is imperative. Aforesaid would be an ideal scenario for dewatering the trench lateral.
To extrapolate the rate of LFG recovery for the pump test, the trenches with a pump installed in 2016 and the section numbers were graphed. Monthly mass balance data begins from Jan. of 2015 to give at least a year period to observe the periodical changes in the trench lateral behavior. The average of the total methane change from the trenches with the pumps installed in 2016 is used to project the methane recovery for the trenches recommended to be dewatered.
The grouped trenches MMB analysis is not equivalent or the reflection of each trench MMB.
5.3 MMB Modeling Results
The Trenches w/ Pump Installed in 2016
a. The trenches with a pump installation in January were sorted and the result is shown in Figure 12. Prior to pumping, the data go back to January 2015 and the average methane flow rate was 22.642 cfm per month. After dewatering with a pump, up-to date an average of methane flow is 55.261 cfm, 244.08% increase in comparison to aforementioned.
b. A pump installed in February was sorted and distributed on a monthly mass balance graph as seen in Figure 13. Prior to pumping, the average of methane production from this lateral was 81.965 cfm per month whereas after pump was installed, the methane production averaged at 152.43 cfm per month, an increase of 185% since dewatering.
c. As for March, methane gas flow before the pump installation averaged 11.00 cfm per month. On the other hand, since dewatering, methane gas flow averaged 26.67 cfm per month, an increase of 243% to aforementioned as seen in Figure 14.
d. In the month of April, the methane gas flow prior to pumping averaged 21.02 cfm per month whereas, after the pump installation, the methane gas averaged 108.4 cfm per month. This group of trenches improved the methane generation by 515.6% as seen in Figure 15.
e. For May, the methane gas flow after dewatering averaged 33.276 cfm per month whereas, it only averaged 11.416 cfm per month before pumping the liquid. Dewatering the trench lateral recovered the methane flow by 291.4% as seen in Figure 16.
f. In the month of June, prior to dewatering, this group of trenches averaged around 34.02 cfm per month of methane flow. After pumping the liquid, the methane gas flow averaged 41.39 cfm in the proceeding two months. A pump improved the methane flow by 121.66% as seen in Figure 17.
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g. For the July group, prior to pumping liquid, the methane flow averaged 42.673 cfm per month. After the pump installation, in August, the methane flow read at 60.43 cfm per month which is an improvement by 141.61% as seen in Figure 18.
h. Prior to the pump installation, the methane flow averaged 24.162 cfm per month and the month of August, the methane flow read at 69.64 cfm as seen in Figure 19. There aren’t sufficient data to determine whether pumps have a positive impact on improving the methane flow to this group of trenches. Further observation should be advised.
The Trenches w/ Pump Installed in 2016 and by the Section Numbers
i. There were three trenches from section 38 had a pump installed in 2016 and the result is a clear sign of recovery of methane flow than the year of 2015 as seen in Figure 20. All three trenches had a pump installed in different month of 2016. Therefore, the data could not be assessed.
j. As for the sections 85 and 86 with pumps, the methane flow declined towards the end of 2015. Currently, the flow is recovering and hovering around 60 cfm per month as seen in Figure 21
k. In the 87 section series, a pump installation made an improvement of methane flow in comparison to a previous year as seen in Figure 22
l. Among all the MMB models, only 89 section group displayed the degeneration of methane flow after the pump installation as seen in Figure 23. Two 89’s had a pump installed in July; hence, more observation should be made before making a comparison to the year of 2015.
With an exception on section number 89, the rest of the MMB models indicated that the flooded trenches with pumps have improved and recovered the methane flow rate as described above. Since these results are grouped trenches, each trench may differ on the MMB models.
5.4 An Estimation of Methane Recovery by MMB Analysis
After accounting trenches by the section numbers and the date of pump installation, the expected methane recovery can be estimated on the flooded trenches that would benefit from dewatering.
There is a total number of 29 trenches with pumps installed in 2016 and by a monthly mass balance analysis, the monthly group trenches made an improvement in methane generation (see Table 1). In terms of the methane flow rate consistency, with exceptions on June and July trench groups where there weren’t enough time periods to consolidate consistency, all others were continuously maintaining or improving the methane flow after installment of a pump. The consistency in this context refers to a continuous positive net flow for more than two months.
As with the trenches grouped by the section numbers, all section trenches displayed an improvement in gas quality and consistency (see Table 2), with an exception of section 89. For example, the methane percent improvement was substantial for the section 38. To calculate the improved methane flow from this section, the first month of pump installation was chosen. Prior to that month would show the behavior of methane flow before dewatering, whereas the subsequent month would display the performance of
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methane collection under a pump. Based on this standard, the section 38 had an increase of 7500% methane recovery. This quantitative approximation can be utilized on the projection of methane recovery for the flooded trenches in section 38.
Based on the video survey of the trench inspections, there were 35 trenches flooded with liquid and the report recommended 22 trenches to have pumps for dewatering. As of today, 8 trenches have pumps installed and 14 trenches still remain for installations. The 8 trenches with pumps have shown a sign of strong methane recovery and this group has reached the highest methane content of gas quality in August as seen in Figure 25. The projections can be made for the remaining 14 trenches based on this outcome. Looking at the result from Figure 25 and the MMB analysis by monthly pump installation in 2016; all 14 trenches should have a pump placed with an expectation of 212.62% increase in methane flow and 100% assurance of consistency in the upcoming months. Moreover, the MMB analysis by the section numbers confirmed the following: 38-195 & 87-005 trenches will have a positive inclination from pumping out liquid with an exception of 88-285. All other trenches are summarized in Table 3 and the projected graph as seen in Figure 24.
5.5 MMB Result for Deformed Lateral
Figure 26 is the MMB model of damaged trench laterals. The methane flow is constantly in a downtrend and the performance of these gas collection systems hit zero twice in the year 2016. The methane flow is clearly in decline and there are high possibilities of these trenches going dysfunctional in the coming months.
6. Recommendation
According to the MMB analysis, the return rate of the methane flow will double after the pump installations. Thus, the pump installations will be beneficial to the PHLF by increasing the methane flow from the horizontal gas collection system. Consequently, the pump installations are recommended for the remaining 14 trenches, 8 currently pumping liquid. Regarding the deformed trenches, the MMB model evidently displays a fluctuating reduction of the methane flow in 2016. Abandoning the trenches would be cost-efficient than fixing the piping; such action will save additional cost in man power and the resources involved in the trench lateral maintenance. Also at the end of the each video survey, there is a recommendation section to be read. Moreover, the recommendations are described concisely in the comment section.
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FIGURES
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