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TERTIARY TREATMENT TECHNOLOGIES & PRACTICES Current Best Practices for Chemical Phosphorus Removal BY JEREMY KRAEMER, PH.D., P.ENG., CH2M Wastewater treatment plants (WWTPs) in Ontario have been required to reduce phosphorus (P) to at least 1 mg/L since the 1970s, with effluent limits down to 0.3 to 0.5 mg/L becoming common through the 1990s. Currently, effluent limits of 0.1 mg/L or less are frequently required when plants are expanded, particularly inland where receivers are often Policy 2 with respect to phosphorus (P). Except for a handful of biological phosphorus removal WWTPs, P removal in Ontario is achieved by chemical treatment typically using ‘alum’ (aluminum sulphate) or ‘ferric’ (ferric chloride or ferric sulphate). A few plants still use ‘ferrous’ (ferrous sulphate, sometimes referred to as pickle liquor), however it requires oxidation before it becomes effective and its use is in decline and will not be covered in this article. This article will discuss current best practices the reader can employ to remove phosphorus to low levels while minimizing alum or ferric use and associated cost. Types of phosphorus To understand P removal we must understand the types of P in wastewater. Although P occurs naturally in many different forms (organic P, ortho-P, polyphosphates, etc.), generally we are interested in two functional categories: FIGURE 1 • Reactive versus Non-Reactive: When P is reactive it means the P can react with coagulants and form a particulate, whereas non-reactive P does not react with coagulants. At WWTPs that are achieving ultra-low effluent P concentrations, it has been observed that there is typically about 0.01 to 0.02 mg/L of soluble non-reactive phosphorus in wastewater effluent. • Soluble versus Particulate: P must be in a particulate form in order to be removed by filters or sedimentation. Being ‘particulate’ is an arbitrary definition based on whether a substance is retained or passes through a total suspended solids (TSS) filter, which has a nominal pore size of 1.5 microns. Substances that are retained are said to be particulate while those that pass through are said to be soluble. However, it is important to note that “soluble” substances could still be either truly soluble (like orthophosphate) or colloidal (like organically-bound phosphorus). The P removal two-step process Chemical phosphorus removal at WWTPs can be simply considered as a two-step process, as illustrated in Figure 1 and outlined below: Step 1 – Convert soluble P to particulate form. In this step we convert Chemical phosphorus removal in two simple steps. reactive P into a chemical particulate (solid) using a coagulant. Step 2 – Remove the particulates. In this step, we use gravity sedimentation or filtration to separate the coagulated solids from the treated effluent. A WWTP’s effluent phosphorus limit then dictates to what degree each step is required; that is, how much soluble P needs to be converted to particulate (and correspondingly how much coagulant needs to be added) and how good a job needs to be done at removing the particulates (i.e., need for tertiary treatment, and if so, what type). These two steps of P removal are further discussed below, with an emphasis on Step 1. STEP 1: Convert soluble P to particulate form (i.e., coagulation) Recent research has significantly improved our understanding of how chemical coagulation works to remove P from wastewater. Notably, this work has a local link: Prof. Scott Smith from Laurier University (see In the Spotlight in this issue of Influents) was part of a team that formulated a new conceptual model of chemical P removal. As part of that work, they published a paper (Szabo et al. 2008) entitled Significance of Design and Operational Variables in Chemical Phosphorus Removal in WEF’s journal Water Environment Research. That paper is a worthwhile read for those who design or optimize chemical phosphorus removal systems. Several key observations from that work are discussed here. Type of coagulant The research showed that alum and ferric coagulants are equally efficient at phosphorus removal on a molar basis i.e., 1 mole of aluminum (Al 3+ ) is just as effective as 1 mole of oxidized iron (Fe 3+ ). Based on typical Greater Toronto Area bulk purchase prices of about $4.50 / kg as Al and $2.00 per kg as Fe, 38 INFLUENTS Fall 2015 Click HERE to return to Table of contents
TERTIARY TREATMENT TECHNOLOGIES & PRACTICES the cost per mole is pretty much equal (12 cents per mole of Al and 11 cents per mole of Fe, using the example costs here). So, the choice of coagulant really comes down to whether your local cost and availability for the two coagulants differ substantially, operator preference (some operators prefer alum because it is less corrosive compared to ferric), and other factors (e.g., ferric helps control sulphide gases in digesters and improves dewaterability compared to alum). Importance of mixing One of the important findings of the research was to identify the importance of rapid mixing of coagulants at the point of dosing into wastewater. This is illustrated in Figure 2, which illustrates that the residual soluble P after coagulation decreases as your coagulation mixing intensity G-value increases (the G-value is the mean velocity gradient, a measure of mixing intensity with units of inverse seconds [1/s or s -1 ]). Ideally, a G-value of at least 300 to 400 s -1 should be provided. In particular, the research found that the first 30 seconds to 1 minute are the most critical for obtaining the most efficient use of the coagulant. When coagulant is dosed into water, it immediately begins forming metal hydroxides that begin to stick together. These initial metal hydroxides that are formed are the most effective at adsorbing P. As the metal hydroxides continue to stick together, ultimately forming visible flocs, most of the useful adsorption sites quickly become ‘lost’ as they become surrounded by other metal hydroxides therefore becoming the internals of the visible chemical floc (we say that P has become occluded inside the floc). Any internal adsorption sites are generally no longer accessible to P that is still in the bulk wastewater. This research tells us that if you want to decrease your coagulant usage, you want your coagulant and your phosphorus to be in close proximity. This explains why higher initial mixing energy improves phosphorus removal as was shown above in Figure 2: higher mixing energy ensures greater contact between P and the early formed metal hydroxides which results in more P being occluded on the inside of the resulting flocs, which maximizes coagulant efficiency (you can do more with less). Conversely, when coagulant just reacts in water to form metal hydroxides without any phosphorus nearby, the resulting ‘pre-formed’ metal hydroxides are much less efficient at P removal than the young and vigorous metal hydroxides formed within the first 30-60 seconds. As examples, pre-formed metal hydroxides can result when you have poor mixing or no mixing at the point of injection, or your Phosphorus Concentration, mg/L 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 FIGURE 2 Higher coagulation mixing intensity improves phosphorus removal (after Szabo et al. 2008). 0 0 50 100 150 200 250 300 350 400 450 Mixing G-Values, s -1 coagulant is mixed with carrier water prior to injecting into wastewater. Contact time Although pre-formed metal hydroxides are not as efficient as freshly formed ones, pre-formed metal hydroxides do still remove P but just at a much slower rate, as shown by the shallow sloped portion of the line in Figure 3. This slow rate is still useful if we can let the pre-formed flocs stay in contact with P for a long time. This is achieved through co-precipitation in the bioreactor. The pre-formed chemical flocs, when part of the mixed liquor, stay around for a time equal to the secondary treatment solids retention time. For example, for nitrifying systems in Ontario this is Commissioning begins at Norfolk County’s Delhi Wastewater Treatment Plant Commissioning has begun at Norfolk County’s Delhi Wastewater Treatment Facility. The $15M project included the construction of a new 3182 m 3 /day plant on an existing site, while maintaining treatment operations. Some temporary works were installed to facilitate the staging during construction and the existing plant will be removed when the new plant is complete. R.V. Anderson Associates Limited was able to avoid the construction of a new wet well by making use of the available grade change at the site. Poor soils and high groundwater levels were additional challenges that were overcome during construction. Click HERE to return to Table of contents WEAO INFLUENTS In uents Fall Magazine 2015 39 7” x 3.25” August 2015
Page 1 and 2: FALL 2015 • VOLUME 10 Send undeli
Page 3 and 4: TOGETHER, MEETING THE CHALLENGES OF
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