Using the Capillary Suction Time Test as a Tool to Support Operation of the Dewatering Process

Introduction The capillary suction time (CST) test is an established method of evaluating the dewaterability of stabilized wastewater solids and has been used primarily to evaluate different flocculants, flocculant demand, or to determine the differences in dewaterability between solids generated under differing conditions. Many variables exist in modern dewatering processes which offer opportunities for optimization. Flocculant make down systems are now tunable with regard to mixing energies, make down concentrations can be easily changed, and the system may or may not include an aging or feed tank. Often times there are options to dose flocculant at multiple points in the solids feed ahead of dewatering equipment. Feed solids concentration and rheology can also have an effect and pre-dilution of the feed offers another control point to gain efficiency. Clean Water Services (CWS) (Washington County, Oregon, USA) is undergoing the development of CST testing procedures to evaluate, predict, and recommend full-scale dewatering process optimization strategies. Procedure The CST apparatus itself is a simple device that measures electrical continuity between two sets of contacts. The contacts rest on a piece of filter paper, a conditions solids sample is applied to the center of the filter, and water is drawn out across the paper. A timer is initiated when the water reaches the first set of contacts at radius 1, and stopped when it reaches the second set of contacts at radius 2. The lower the time the more easily water is withdrawn from the solids, and the more dewaterable the sample is considered to be. CWS utilizes an OFITE (Houston, Texas, USA) CST apparatus (Part No. 294-50). Although simple in concept, the CST test requires a well thought out procedure and careful technique to improve the accuracy and repeatability of the results. The manufacturers instruction manual contains procedure recommendations including using a syringe or pipette to transfer the conditioned sample to the sample holder, thoroughly cleaning and drying the sample blocks and electrodes between tests, and repeating a minimum of three times and averaging results. Additional notes state that temperature and solids concentration have significant effect on results, and that 5 repetitions per test are recommended. The consequence of these procedural nuances is that it will take a significant amount of time to evaluate a single solids sample at a single flocculant dose through 3-5 repetitions. Over the course of the time required, two major effects can be seen to skew results. First, when evaluating anaerobically digested solids the initial sample will be at a temperature of approximately 37 C for a mesophilic system. Depending on sample size and ambient lab temperature the sample may cool as much as 10 C over the run of 5 repetitions. Second, the made down flocculant solution, whether prepared in the lab or taken from the full-scale system will be aged up to one hour over the trial run. The goal with any bench test is to duplicate as exactly as possible the full-scale system. The procedure under development at CWS attempts to achieve this in multiple ways. The test is considered to be a field test, meaning that it is performed as close to the dewatering process as possible. The flocculant solution is sampled as close to the dosing point as possible to account for activity changes in the piping run from the make down system to the dosing point. The solution is used in the test as quickly as possible, ideally under 1 minute, and a fresh sample is taken for each test. A 4 L dewatering feed sample minimizes temperature changes. In the test itself a 60 mL syringe is capped and inverted, plunger removed, and a 5 mL sample of solids and corresponding polymer dose are concurrently pipetted into the syringe. The plunger is inserted enough to make a seal, the syringe is placed tip up, the cap loosened and the plunger depressed to a volume of 30-40 mL. The cap is tightened and syringe is shaken 30 times in a back and forth motion to effect flocculation. The cap is removed and the conditioned solids are pressed to the end of the syringe, then displaced into the sample holder. Results Initial results of procedure development and investigation of effect of flocculant solution aging illustrate the inherent variability in the test and increases in CST time from flocculant that has been aged on the lab bench. In Figure 1, a dose curve of CST time is shown for the same solids sample conditioned with the same fresh flocculant and run on side by side CST instruments. Although the curves have the same shape, there is chance variability in the CST at a dose of 14 lb. active/DT. The effect of solution aging can be pronounced as shown in Figure 2. Here, two separate runs are shown where the dose remains at 20 lb. active/DT but the solution has aged on the bench up to 30 minutes. Summary The CST test may prove to be a useful tool to support operational decision making and to evaluate the effect in dewaterability of solids treatment processes. The method is however, sensitive to technique and procedure and the nuances of the analysis as a representative bench test must be considered if the resulting data are to be useful. The future of this work, to be completed by time of manuscript include a full procedure, evaluation of effect of flocculant aging, solids sample temperature, flocculant solution concentration, and dosing point. These data will be accompanied with a correlation to full scale operational performance.