Municipal wastewater treatment costs with an emphasis on assimilation wetlands in the Louisiana coastal zone

Highlights

• Cost for assimilation wetlands averaged $0.60/gallon of treatment capacity.

• 2° and 3° treatment costs averaged $4.90 and $6.50/gallon, respectively.

• Wetland assimilation is less sensitive to cost increases than traditional systems.

Abstract

In recent decades, water quality standards for wastewater treatment have become more stringent, increasing costs and energy required to reduce pollutants. Wetland assimilation is a low-cost and low-energy alternative to traditional tertiary wastewater treatment where secondarily treated and disinfected municipal effluent is discharged primarily into freshwater forested wetlands in coastal Louisiana. In this paper, costs per gallon of treatment capacity for conventional secondary and tertiary treatment were compared to those for assimilation wetlands. Cost analysis reports were used to determine costs per gallon of treatment capacity for conventional wastewater treatment facilities, including costs for conveyance between the collection system and the assimilation wetland site, and between the treatment and disposal sites if they could not be co-located. Capital and operation and maintenance costs were considered. Because all wastewater treatment plants are required to treat at least to secondary standards, costs for primary and secondary treatment were combined. If necessary, these costs were adjusted for inflation to 2017 dollars using an average inflation rate of 2.19 percent and a cumulative inflation rate of 50.84 percent. To determine costs per gallon of treatment capacity for assimilation wetlands, actual costs provided by the project engineer were used when available. To simulate the future costs of facility construction and compare the replacement costs of conventional secondary and tertiary wastewater treatment facilities and treatment wetlands in the context of energy prices, U.S. Bureau of Labor and Statistics (BLS) data for the price index for inputs to construction were used, as were the Energy Information Administration (EIA) data for the price of crude oil to model future wastewater treatment plant construction and operation costs. The cost for the Mandeville assimilation wetland included $1 million for the price of the land. Future costs of treatment facility construction and operation were modeled relative to average price of construction inputs between 1998 and 2015 using the projected price of crude oil. When treatment costs were compared among secondary, tertiary, and assimilation wetlands, mean cost for assimilation wetlands was $0.60 per gallon (>1 MGD capacity) compared to $4.90 and $6.50 per gallon for secondary and tertiary treatment, respectively. The lower total costs and energy requirements for assimilation wetlands result in lower variability in the price of construction and operation. Wetland assimilation is more economical than conventional wastewater treatment, especially compared to advanced secondary and tertiary treatment. It is likely that energy costs will increase significantly in coming decades. Because conventional secondary and tertiary treatment are energy intensive, increases in energy costs will significantly increase the costs of these treatment systems. Treatment systems that combine lower technology (e.g., oxidation ponds) secondary treatment with wetland assimilation are less likely to be impacted by rising energy costs than traditional wastewater treatment.

Introduction

Conventional treatment of municipal sewage is energy and capital-intensive. Over the past half-century, water quality standards for effluent discharge have become progressively more stringent to address pervasive water quality problems. More stringent regulations have resulted in improvements in water quality but also increased costs per gallon of treatment capacity, especially for smaller municipalities where unit treatment costs are higher than for larger ones (USEPA, 2015). Assimilation wetlands are natural wetlands into which secondarily treated and disinfected municipal effluent is discharged (Hunter et al., 2018, Day et al., 2018), removing nutrients at a much lower cost than conventional tertiary treatment (Godfrey et al., 1985, Kadlec and Wallace, 2009, Nagabhatla and Metcalfe, 2017, Ko et al., 2004). Selection of treatment methods for nutrient removal is increasingly important as more stringent discharge limits are placed on municipal and industrial dischargers. In Louisiana, the majority of dischargers with a Louisiana Pollutant Discharge Elimination System (LPDES) permit do not have nutrient limits for their effluent. However, as eutrophic concentrations of water bodies increase, nutrient limits are becoming more common, as well as biological oxygen demand (BOD) and total suspended solids (TSS) limits below the typical limits of 10 and 15 mg L⁻¹, respectively.

There are three levels of municipal wastewater treatment, with each achieving a greater reduction in BOD, TSS, and nutrient concentrations (Hartman and Cleland, 2007). Primary wastewater treatment can reduce BOD by 20–30 percent and suspended solids by up to 60 percent. Secondary treatment incorporates biological processes to further remove dissolved organic matter and additional settling processes to further reduce suspended solids. Secondary treatment can remove up to 85 percent of BOD and TSS. Tertiary treatment reduces nitrogen and phosphorus concentrations of secondarily treated effluent. Tertiary treatment can reduce total nitrogen (TN) and total phosphorus (TP) concentrations to as low as 3 and 0.3 mg L⁻¹ or less, respectively, depending on the treatment process utilized (Hartman and Cleland, 2007, Kadlec and Wallace, 2009). These processes rely on microbial activity to reduce nitrogen and phosphorus along with chemical and physical processes.

Nitrogen in secondarily treated municipal effluent is generally in the form of ammonia and organic nitrogen and it is typically not significantly removed by conventional secondary treatment. Secondary treatment with a high degree of aeration can convert ammonia to nitrate. Transformation of nitrogen is achieved through a series of biochemical reactions that transform nitrogen from one form to another, with key transformations being nitrification and denitrification (Reddy and DeLaune, 2008). During conventional wastewater treatment, phosphorus can be removed through chemical precipitation or physical processes using filtration and membranes but removal is generally less than 20 percent. Chemical precipitation produces a sludge, and the cost of disposing of this material can be significant (Keplinger et al., 2004). Enhanced biological phosphorus removal typically involves an activated sludge process modification (alternating aerobic and anoxic conditions) that allows for a high degree of phosphate removal from wastewater, with the potential to achieve very low (<0.1 mg L⁻¹) TP effluent concentrations (Hartman and Cleland, 2007). Assimilation wetlands can reduce TN and TP to background levels so that there is no net input to open water systems (Day et al., 2004, Day et al., 2018, Hunter et al., 2018).

Conventional wastewater treatment is very energy intensive and the costs of operations, maintenance and construction are tightly correlated with energy prices (Bodik and Kubaska, 2013). When calculating life cycle costs for wastewater treatment, the initial cost for facility construction must be considered in conjunction with operation and maintenance costs and future replacement costs. Conventional treatment facilities generally have an operational life of 30–40 years and suffer declines in efficiency (and/or increases in costs) during later years due to several factors such as natural wear and tear, equipment type and materials, and lack of preventative maintenance, primarily because of the highly technical and mechanical nature of “concrete and steel” facilities (Vigneswaran, 2009). Assimilation wetlands have essentially unlimited operational lives, and forested wetland sites have the additional benefit of potential selective timber harvesting (Kadlec and Wallace, 2009, Hunter et al., 2018, Day et al., 2018). In this paper, facility wastewater treatment costs and costs for discharging into an assimilation wetland (price per gallon of treatment capacity ($GTC)) are discussed, along with estimated future costs based on rising energy costs.