WEF Collection Systems Conference Evaluation of High ph Infiltration Water and Recycled Bedding Material

Transcription

1 Evaluation of High ph Infiltration Water and Recycled Bedding Material Keith Hobson, PE, BCEE, FOX Engineering Associates, Inc. ABSTRACT The purpose of this evaluation was to determine the cause and source of high ph (alkaline) wastewater in the CBI Southlands sanitary sewer in Council Bluffs, Iowa. This newly installed 48-inch sanitary sewer in south Council Bluffs filled with a high ph wastewater and the first discharge of this wastewater to the City wastewater treatment plant created a resultant rise in the ph of the total wastewater stream. A recycled crushed concrete material was used for the bedding support during the pipe installation. A number of water and experiment samples and tests were conducted to identify the alkaline material and source potential. The potential that something in the pipe bedding could cause such an increase in ph and calcium through infiltration through leaking joints into the piping was explored. The results of testing are presented that shows that the calcium levels were over 2,000 mg/l as CaCO 3 in the distilled water. This is similar to the process that occurs in a lime softening water treatment plant. Even though the recycled concrete had the same structural strength and pipe lateral support as natural rock, it did not prove to provide the same long lasting support. Based on the testing and confirmation of high alkalinity wastewater the sewer line was completely removed and replaced. The new installation was completed with new bedding material of natural rock to avoid this situation. KEYWORDS High alkalinity wastewater, recycled concrete, pipe bedding, lime softening chemistry, and sanitary sewer. INTRODUCTION A new sanitary sewer, pumping station, and force main were installed in Council Bluffs, Iowa in 2012 to serve a new industrial park in the southern part of the city. The site is also within one mile of the Missouri River and pipe excavation was in sand and loess material. Dewatering was required during construction to lower the water table in the area. This sanitary sewer portion consists of 48-inch diameter fiberglass-reinforced, polymer mortar pipe made by HOBAS and 21-inch diameter PVC pipe. The pipe was installed with bedding material consisting of recycled crushed concrete to simulate crushed rock. After installation this sewer was idle for several months after leakage testing. During this period a certain amount of ground water had entered the pipe. When this water was discharged to the City wastewater treatment plant it created a resultant rise in the ph of the total wastewater stream at the plant creating a major concern. Figure 1 below illustrates the rise in ph of the plant influent from an average of about 7 to over 9 with the new sewer discharge. The new sewer 578

2 discharge was terminated until the source of this high ph wastewater could be determined and treated. This newly installed sanitary sewer from the CBI Southlands Industrial site in south Council Bluffs had apparently filled with a high ph wastewater. Figure 1. Council Bluffs Water Pollution Control Plant influent flow and ph levels during the first sewer discharge on January 25, FOX Engineering was hired to investigate this situation and determine the cause and potential treatment. The purpose of this evaluation is to determine the cause and source of high ph (alkaline) wastewater in the CBI Southlands sanitary sewer and determine appropriate treatment methods and prevent further contamination. METHODOLOGY In order to evaluate the source of the high ph wastewater, we held discussions with a number of site personnel and visited the area to check on potential sources. Discussions were held with city wastewater staff and city engineering staff, city water works staff, sewer installing contractor staff, construction observers, geotechnical engineers, and area industries. We then reviewed the site of the sewer and potential water and contamination sources. We requested a number of water and experiment samples and testing to be conducted to identify the alkaline material and source potential. We also researched a number of papers and reports on related alkaline sources. Based on the sewer layout, topography, and sewer tributary area a number of potential sources were identified for the alkaline contamination including: o Potential Source #1 Area industries and construction site o Potential Source #2 Illegal discharge from hauler into area manhole o Potential Source #3 Groundwater infiltration and surface water inflow These are evaluated in the following sections. 579

3 SOURCE EVALUATION Industrial Site Observation FOX staff visited the industries in the area that were connected to the sewer to observe construction activity, water sources, and chemical storage within the buildings that had been connected to the sewer. A record of the potable water supplies to the sites was also reviewed to compare to the amount of sewage discharged. Based on this review of operations and chemical storage, there was not sufficient quantities of any chemicals present to produce the high levels of ph and alkalinity in the wastewater pipeline. Water Chemistry Review The measured ph in the wastewater contained in the sewer line was As indicated earlier a number of water tests were conducted to identify the source chemicals. These samples were taken at various manholes throughout the sewer line and tested for typical inorganic parameters as presented in Table 2. A complete organic toxicity test was also completed on one sample to check for other potential contaminants. However, no potential toxic levels of organic compounds were found in the testing. Additional tests were then performed on constituents believed to contribute to the alkalinity. Potential causes of high ph water in the sanitary sewer that were explored as part of this evaluation include the following: 1) High ph Due to Lime Softening Chemistry within Pipe Bedding 2) High ph Due to Strong Base Addition Due to Waste Dumping A more detailed discussion is included below. 1) High ph Due to Lime Softening Chemistry within Pipe Bedding a. Background Lime Softening in Drinking Water Lime softening is a conventional drinking water treatment process that involves the removal of calcium and magnesium (total hardness) by increasing the ph of the water and subsequently precipitating calcium as calcium carbonate and magnesium as magnesium hydroxide. In general, the calcium hardness is removed (precipitated and settled) by raising the ph to 10.3, while the magnesium hardness requires a higher ph of around 11. The ph is raised through the addition of some form of lime which may be either calcium oxide (quicklime, CaO) or calcium hydroxide (hydrated lime, Ca(OH) 2. After lime addition, the raw water is mixed and the white hardness precipitate is formed, flocculated and settled in a matter of hours. Because the precipitates are very slightly soluble, some hardness remains in the water typically on the order of mg/l as CaCO3 for drinking water. The total hardness of a water is the concentration of multivalent cations in water such as calcium, (Ca 2+ ), Magnesium (Mg 2+ ), iron, manganese, strontium, barium, zinc, aluminum and other ions. Total hardness is typically defined as simply the sum of calcium and magnesium hardness, since 580

4 the other multivalent ions are generally not present in significant quantities. The total calcium (i.e. hardness) measured in the sanitary sewer samples was much higher than would be expected from a typical source of groundwater infiltration which raised a red flag and question regarding the calcium source. This issue is addressed in the following section. b. Lime Softening in Recycled Concrete Bedding The chemistry which occurs in a conventional drinking water plant with lime softening treatment was observed to be present in the samples collected from the sanitary sewer. The samples exhibited a very high ph and extremely high calcium (i.e. hardness) levels which are well above (2-5 times) the anticipated hardness levels in fresh/shallow groundwater that could infiltrate the pipe. Table 1. Typical Hardness Levels in Water Water Type Total Hardness Notes (mg/l as CaCO 3 ) Soft 0 to 75 Moderate 75 to 150 Typical Softened Drinking Water Quality Hard 150 to 300 Typical Fresh Water/Shallow Ground Water mg/l Typical Council Bluffs Raw Sewage mg/l Very Hard >300 Iowa Ground Waters/Aquifers mg/l The potential that something in the pipe bedding could cause such an increase in ph and calcium through infiltration through leaking joints into the piping was explored. In order to confirm or deny this theory, a sample of the recycled concrete pipe bedding was collected and distilled water was poured into the sample and allowed to set for 24 hours. After that time, there was a white precipitate in the sample. The liquid was poured off and sent to a laboratory for testing and the results are presented in the second column in Table 2 below. The results are also illustrated in Figure 2. The calcium levels were over 2,000 mg/l as CaCO 3 in the distilled water which shows that there is a strong potential for ground water in contact with the recycled pipe bedding to become saturated with calcium as it flows through the bedding. This results in a gain in alkalinity and increase in ph. This is similar to the process that occurs in a lime softening water treatment plant. The fact that the measured sewer samples had elevated calcium, but not elevated magnesium as in lime softening required additional analysis. The distilled water soaked in the recycled concrete bedding had an elevated magnesium level, yet there was virtually no magnesium in the sewer samples. As a potential explanation, it was previously noted that calcium carbonate and magnesium hydroxide floc are quite different. The calcium carbonate floc is comprised of dense crystals that are negatively charged and are benefited by the use of a coagulant aid for settling. Calcium carbonate (CaCO 3 ) removal is aided by a coagulation/flocculation process to grow the 581

5 particles to induce settling. A mechanism for growing the CaCO 3 particles would not be present within the pipe bedding and it is theorized that a portion of the fine CaCO 3 particles would make their way through the bedding and into the piping. Magnesium hydroxide floc, on the other hand, is similar to a coagulant aid such as alum (aluminum sulfate) which has positively charged sites that can neutralize the charges of particles that allow them to clump together. It is again theorized that the Magnesium Hydroxide particles might be more easily retained within the pipe bedding and would not pass through into the sewer pipe. The other theory is that the shallow ground water had a very low Magnesium level to begin with. Testing results also showed that the recycled concrete significantly added potassium and sodium to the water which was also observed to be high in the sewer samples. This is another indicator of the potential for groundwater into the pipeline to be the source of high ph water. Table 2. Measured Hardness Levels in Water Chemical Original Sample Collected (Lift Station-MH Bypass) Distilled Water w/ 24- Hr Soak in Recycled Concrete Bedding MH#2 Sample Collected (Lift Station-MH Bypass) Pipe Seepage Sta Collected Total Alkalinity, mg/l as CaCO Total Calcium, mg/l 568 2, mg/l as CaCO3 1,420 5, Chlorides, mg/l Hardness (Total), mg/l as CaCO3 >1,420 est. (1) 5,850 est. (1) 594 est. (1) 822 Iron, mg/l Total Magnesium, mg/l mg/l as CaCO3 Manganese, mg/l n.d. (2) ph Potassium, mg/l Sodium, mg/l Sulfates, mg/l Total Dissolved Solids 4, ,972 (TDS), mg/l Total Suspended Solids (TSS), mg/l (1) Total hardness estimated from the sum of the calcium and magnesium. (2) n.d.= not detected 1,

6 Several other studies (Thiele Geotech 1996 and Steffes 1999) have been conducted on recycled crushed concrete material and similar observations were made regarding reaction of the material with ground water to produce high ph water and resultant precipitation of calcium carbonate. Therefore it was thought that the high ph wastewater could be produced from the recycled concrete bedding material. 2) High ph Due to Strong Base Addition Due to Waste Dumping The potential for the ph to be raised due to the dumping of a strong base was explored, since only a strong base would be capable of raising the ph to levels above 12. A list of strong bases was considered and the most common commercially available strong bases outside of hydrated lime were considered to be sodium hydroxide and potassium hydroxide. The potential for the ph to be raised due to the dumping of a strong basic solution such as Sodium Hydroxide (NaOH) or Potassium Hydroxide (KOH) was explored since these strong bases would be capable of raising the ph to the measured levels and also due to the presence of sodium and potassium in the samples. Based on results of the Rothberg, Tamburini and Winsor Model for Corrosion and Water Chemistry (RTW3.0 Model), it was estimated that a dosage of over 500 mg/l of sodium hydroxide into ground water would be required to increase the ph of a typical groundwater from 8 to This dosage would result in a sodium concentration of over 280 mg/l which is approximately three (3) times the measured values, so there is not enough sodium in the water to indicate that NaOH is responsible for raising the ph. The potential for KOH dumping was explored and similar results were determined indicating that over 500 mg/l of KOH would be required as well. This dosage would result in a potassium concentration of over 350 mg/l which is approximately five (5) times the measured values, so there is not enough potassium in the water to indicate that KOH is responsible for raising the ph either. Therefore, chemical analysis showed that the concentrations of the two strong bases of sodium hydroxide or potassium hydroxide were not high enough to cause the observed ph. This ruled out the likelihood of a large chemical dump to cause the high wastewater ph issue. 583

7 Figure 2. Results and graph from lab testing of distilled water and recycled concrete pipe bedding showing elapsed time and ph. 584

8 DISCUSSION AND CONCLUSION The observations of the wastewater characteristics and the water testing indicated that the recycled crushed concrete bedding was reacting with the groundwater and producing a high ph water that infiltrated the sewer and precipitated out calcium carbonate. This created not only a high ph wastewater but also one high in total dissolved solids (TDS) and total suspended solids (TSS). In order to discharge this waste to the city the contractor was required to neutralize this waste with acid to bring the ph down to 9 before discharging. A procedure was developed for removing the wastewater, neutralizing with acid in fracking tanks and then discharging the equalized waste to the city for further treatment. Even though the crushed concrete material was adequate for strength and abrasion as a bedding material and of correct gradation, the material was not deemed acceptable for long term use. In addition to producing the high ph water the bedding material also degraded from the chemical softening process and the leaching of calcium carbonate. Therefore it would lose its structural integrity over time. The recycled crushed concrete bedding material needed to be replaced. In order to correct this situation the complete sewer and bedding material was removed and the pipe was re-laid with natural crushed rock bedding material. For sewer pipe bedding material in groundwater conditions it is recommended that recycled crushed concrete material be avoided as a suitable bedding material. Even though there are benefits to using a reclaimed product and reducing natural resource use, it does not provide adequate long term pipe support and its use creates additional high ph (alkaline) wastewater. REFERENCES Thiele Geotech Inc. (August 20, 1996) Recycled Aggregates in Infrastructure Construction prepared for Nebraska State Recycling Association, Omaha, Nebraska. Steffes, Robert (September 1999) Laboratory Study of the Leachate From Crushed Portland Cement Concrete Base Material produced for the Project Development Division, Iowa Department of Transportation, Ames, Iowa. 585