MAKING THE SWITCH FROM LIME TO MEMBRANE SOFTENING: WHEN IS IT THE RIGHT TIME? Introduction

Transcription

1 MAKING THE SWITCH FROM LIME TO MEMBRANE SOFTENING: WHEN IS IT THE RIGHT TIME? Joseph R. Elarde CH2M HILL 5801 Pelican Bay Blvd., Suite 505 Naples, FL Jeff Poteet, Marco Island Utilities, Marco Island, FL Justin Martin, Marco Island Utilities, Marco Island, FL Mickal Witwer, CH2M HILL, Gainesville, FL Nickolas J. Easter, CH2M HILL, Naples, FL Introduction Several existing lime softening water treatment plants (WTPs) are facing challenges related to higher operating costs due to significant increases in chemical and sludge disposal, as well as increases in operations and maintenance costs as facilities age. When coupled with staffing issues, the need for major capital replacement of existing infrastructure and more stringent drinking water regulations, many utilities look to replacing the lime softening system with nanofiltration (NF) or even low-pressure reverse osmosis (LPRO). The lower operating cost, better treated water quality and increased operating flexibility make an NF/LPRO system very attractive in most cases. However many site-specific conditions are important to consider when deciding to make the switch to a new membrane softening process versus investing in existing lime softening infrastructure. A full analysis of both costs and non-costs factors is needed at each facility to determine the best path forward for a given facility and utility. This paper summarizes the decision process and factors that go into this analysis and provides a real-life example of the analysis results. Decline of Lime Softening Lime softening historically was a lower cost and more robust treatment process for softening hard groundwater and surface water. Figure 1 and Figure 2 show a comparison lime softening versus NF construction and operations and maintenance (O&M) costs respectively in Florida as of At that time, the cost of membrane softening was higher than lime softening, however the gap was not very large. Since that time, lime chemical costs and sludge disposal costs, as well as labor costs have increased significantly, while unit power costs have remained steady or have decreased. Similarly, the construction costs of membrane softening systems have declined and unit power consumption has declined with the introduction of new membrane suppliers into the market place and the advancement of membrane technology respectively. These factors are causing membrane softening to become even more cost competitive or cost advantageous in many instances. 1

2 WTP Construction Cost ($/gpd) Membrane Softening Lime Softening Production Capacity (mgd) Figure 1. Florida Lime versus NF Construction Costs (Bergman, 1995) Figure 2. Florida Lime versus NF O&M Costs (Bergman, 1995) 2

3 The decline of lime softening is also apparent in the non-cost decision factors. Historically, lime has been used to soften hard groundwater supplies that often have color and TOC. Some facilities have added coagulant with the lime or have used split treatment to remove total organic carbon (TOC) and color. Even with these measures, the formation of disinfection byproducts (DBPs) including trihalomethane (THM) and haloacetic acid (HAA) have forced many facilities to switch to chloramine residual disinfection. The switch to chloramines has resulted in problems with color & regrowth in distribution systems and now there is emerging concern over the formation of nitrogenous disinfection byproducts (N-DBPs). Other factors that have resulted in the decline of lime softening include high-doses of lime resulting in residuals handling challenges and advanced credit for pathogen removal has not been granted as with membrane softening. Lime versus Membrane Softening: Non-Cost Factors Table 1 shows a list of major non-cost factors considered when evaluating the implementation of lime softening versus membrane softening. The table shows where the advantage typically falls when comparing the processes. The non-cost factors which discussed in more detail in the sections typically drive the selection of membrane softening. Table 1. Lime versus Membranes Typical Non-Cost Factors and Advantages Non-Cost Factor Treated Water Quality Regulatory Compliance Process Flexibility Ease of Phasing Site Aesthetics Land Requirements Labor / Maintenance Membrane Advantage Lime Advantage Waste Disposal Process Risk/Uncertainty Process Verification Studies 3

4 Treated Water Quality / Regulatory Compliance The membrane process has the advantage of removing TOC including THM and HAA and other DBP precursors that lime softening does not remove. Often, lime softening WTPs use a chloramine residual to control DBP formation, however this method of DBP control may not be sustainable because of distribution system water quality issues including occasional bacteriological growth/nitrification and emerging concerns over N-DBPs. N-DBPs are formed when nitrogen-containing compounds react with oxidants/disinfectants and has been found to be significant in chloraminated water systems with long contact time of precursors with chloramines. N-DBPs are being researched as potential health risks and are suspected to be more toxic than THMs and HAAs. California and Massachusetts have set regulatory levels for nitrosamines while the EPA continues to evaluate future regulatory requirements. In addition to removing hardness at a higher efficiency than lime softening without the need for additional chemical such as soda ash, membranes also have the benefit of reducing other contaminants including contaminants of emerging concern (CECs) that may become an issue for surface water and shallow groundwater treatment facilities as these compounds are further regulated. Many states are also providing enhanced pathogen removal credit to membrane systems that demonstrate sufficient conductivity removal. This removal credit is often easier to achieve than standard removal credit by lime softening that requires meeting filtered water turbidity limits. Process Flexibility / Ease of Phasing Membrane softening provides major advantages to process flexibility because of the insensitivity of the process to changing water quality, the consistency of treated water quality, and the quick turning on and off of the trains to meet demands. In contrast, lime softening is prone to upsets during major changes in flow and feed water quality that impact operations and can have a serious impact on treated water quality. Phasing allows the utility to save some cost for the ultimate build-out of the facility while making the expansion relatively simple when needed. Membrane facilities also have an advantage in that they can be phased to allow for easy future expansion by providing room in the initial building construction for future skids that can be easily installed later. Expansion of lime softening would require major construction of new basins and other equipment. Site Aesthetics / Land Requirements Site aesthetics favor membrane softening because the membrane system, pretreatment, and pumping can be hidden inside a moderately sized building while lime reactors are large structures that are located outside. A larger footprint is required for a lime softening facility because of the need for multiple processes, the need for footprint for settling of the treated water and solids handling facilities. Figure 3 demonstrates how a membrane softening facility better fits into a neighborhood. 4

5 Figure 3. Lime versus Membrane Site Aesthetics Labor / Maintenance The lime softening process is a maintenance intensive process because lime slakers and feed lines require constant maintenance to avoid plugging with solids. The waste solids also require labor intensive management often using a thickener and a dewatering sludge press. The lime reactor clarifiers also require periodic cleaning to remove solids buildup. The lime softening process requires consistent attention to maintain proper chemical dosing. In addition, downstream recarbonation channels and filters require cleaning, media replacement, and maintenance of a significant number of valves. The membrane softening process by contrast, is automated with minimal need for ongoing maintenance. There are less valves to maintain than standard filters and cartridge filters are typically replaced every 3 to 6 months and membrane elements every 5 to 10 years. Waste Disposal Lime softening requires solids handling and disposal. Solids handling is often space and labor intensive and requires significant truck traffic for ultimate disposal. The disposal of solids often requires the facility to rely heavily on outside sources for disposal which gives the owning utility very little institutional control. The benefit of lime softening however is that all of the liquid residuals can be recycled within the process thus eliminating a potential major disposal challenge. Membrane softening by contrast does not produce a significant solid residual that requires handling and disposal. However the liquid concentrate, which is typically 10 to 20 percent of the feed water requires disposal. Typical disposal options include drying/percolation ponds, surface water discharge, sewer disposal, deep well injection and reuse. When looking at a comparison to lime softening that typically treats a source water that is low in TDS that is primarily hardness and alkalinity with low chloride and sulfate. Therefore the concentrate byproduct from the membrane system is typically suitable for wastewater treatment or direct reuse which can turn a potential liability into a potential source of revenue and possibly make disposal cost neutral. However in some locations, concentrate production may become a fatal flaw for the use of membranes because of the high volume of liquid waste that requires disposal. A membrane project may become infeasible if there are concentrated constituents that limit reuse or disposal and there is no location to manage the potentially high volume of waste. 5

6 Process Risk / Verification Studies Lime softening is an established unit process that can treated low to high turbidity surface source waters as well as ground waters. The chemistry is well understood and design based on chemical stoichiometry is reliable. There is little process risk when treating a source water that would otherwise create a potential problem for membrane softening because of organic, particulate or biological fouling potential. While membrane softening has been well established for most groundwater sources, potentially time consuming and costly piloting is often highly recommended if not required for groundwater sources that have turbidity issues or that are under the direct influence of surface water. Piloting is highly recommended for all surface water sources. Lime versus Membrane Softening: Marco Island Case Study Marco Island Utilities owns and operates a water treatment system consisting of two WTPs located on Marco Island. North Water Treatment Plant (NWTP) is 6.67 million gallons per day (mgd) capacity facility, which treats raw water from a lake surface water supply using lime softening and microfiltration (MF). South Water Treatment Plant () is 6.0 mgd capacity RO treatment facility which desalts brackish groundwater from a deep aquifer wellfield located on the island. The existing wells have shown increasing trend in salinity that is driving up the operational cost associated with the RO process. Approximately 3.0 mgd of treated water from the NWTP is transferred to the for blending with the RO permeate before distribution. Figure 4 shows the process flow diagram and average flow balance for the existing facilities. NWTP Alum Lime Hypochlorite Phosphoric Acid 6.00 mgd MEMBRANE FILTRATION Ammonia 3.00 mgd TO DISTRIBUTION SYSTEM MARCO LAKES RAW WATER LIME SOFTENING REACTOR 6.32 mgd 6.00 mgd 4MG STORAGE TANK HIGH SERVICE 3.00 mgd BACKWASH RECYCLE BACKWASH RECOVERY BASIN 0.32 mgd BLEND TRANSFER TO BLEND LINE 2.67 mgd CARTRIDGE FILTERS 2.00 mgd 5.00 mgd TO DISTRIBUTION SYSTEM WELLS Scale Inhibitor FEED RO TRAINS DEGASIFIER CLEARWELL TRANSFER STORAGE TANKS Ammonia Hypochlorite HIGH SERVICE Figure 4. Marco Island Existing Water Treatment Facilities Process Flow Diagram 6

7 An optimization study found that converting to membrane softening at the NWTP will reduce significant annual operating cost associated with lime and sludge disposal, while significantly improving finished water quality to reduce disinfection byproducts, color, hardness, and elevated chloride. The improvements will also significantly improve reliability and operability of the aging NWTP. The proposed membrane treatment will use the existing MF system for pretreatment of the surface water before treatment by new LPRO membrane trains. Figure 5 shows the proposed membrane softening process with flow balance. Pilot testing will be conducted to verify the proposed operating scheme and identify pretreatment modifications needed to prevent biofouling. 0.5 mgd NWTP 8.2 mgd Alum 8.6 mgd MEMBRANE FILTRATION 8.2 mgd Scale Inhibitor TO WWTP or Injection Well Hypochlorite Ammonia TO DISTRIBUTION SYSTEM MARCO LAKES RAW WATER ALUM SETTLING 0.40 mgd 5.0 mgd 4.50 mgd LPRO TRAINS 3.5 mgd 4MG STORAGE TANK HIGH SERVICE BACKWASH RECYCLE BACKWASH RECOVERY BASIN Hypochlorite Ammonia 3.2 mgd BLEND TRANSFER TO BLEND LINE 4.2 mgd 1.1 mgd CARTRIDGE FILTERS 0.8 mgd 5.0 mgd TO DISTRIBUTION SYSTEM WELLS Scale Inhibitor FEED RO TRAINS DEGASIFIER CLEARWELL TRANSFER STORAGE TANKS Ammonia Hypochlorite HIGH SERVICE Figure 5. Marco Island Proposed Membrane Softening Process Flow Diagram In addition to several non-cost benefits of using membrane softening, a cost analysis of the options also shows a less than five-year payback of the capital investment to switch to membrane softening when accounting for some required upgrades to the lime solids handling system that are needed within the next two years. Table 2 shows the construction, operations, and life cycle cost comparison between options to rehabilitate the existing lime softening system and install a new membrane softening system at the NWTP. The membrane softening system will allow the more expensive RO system to operate less while making better use of the NWTP fresh water source. Even with the higher energy requirements of the LPRO system, MIU is anticipating to save more than $1.1M annually in operating costs primarily due to eliminating the cost of lime deliveries, sludge disposal, and maintenance. This cost does not consider potential labor cost savings. The membrane concentrate will be disposed of down the existing deep injection well or may be blended with treated wastewater for beneficial irrigation reuse. If used for irrigation, MIU will realize additional reuse distribution income as well. 7

8 Table 2. Marco Island Lime versus Membrane Softening Project Cost Comparison Item Cost Factor Rehabilitate Lime System Install Membrane Softening Construction Costs Lime System Repairs $1,000,000 Membrane System $2,000,000 Feed Pumps $200,000 Feed Pump VFDs $100,000 Electrical Improvements $100,000 Piping $100,000 Scale Inhibitor Chemical System $30,000 Ammonia System Modifications $5,000 Membrane Cleaning System $50,000 Membrane Building $240,000 Equipment Installation $100,000 Programming & I&C $100,000 Total Installed Cost $1,000,000 $3,025,000 Contractor Overhead 10% $100,000 $303,000 Contractor Profit 5% $55,000 $167,000 Mob/Bonds/Insurance 5% $58,000 $175,000 Contingency 30% $364,000 $1,101,000 Escalation $48,000 $296,000 Engineering / Permitting $82,000 $964,000 Total Project Cost $1,707,000 $6,211,000 Debt Service 5 Year Life Cycle 5 Year Life Cycle Annual Debt Service (4% interest rate) $384,000 $1,396,000 Annual Operating Costs NWTP Chemical $997,000 $296,000 Power $362,000 $610,000 Sludge Disposal $183,000 Membrane Replacement $96,000 $120,000 General Maintenance $80,000 $15,000 Chemical $200,000 $94,000 Power $383,000 $192,000 Consumables $86,000 $83,000 Contingency 20% $478,000 $282,000 Operating Costs (Initial Year) $2,865,000 $1,692,000 Operating Costs (Inflated 3.0%) $3,127,000 $1,847,000 Net Present Value $16,009,000 $15,827,000 8