Sulfur is often considered one of the four basic raw materials in the chemical industry. It can be recovered as a by‐product from sulfur removal and recovery processes (Kirk and Othmer 2004). Historically, sulfur recovery processes focus on the removal and conversion of hydrogen sulfide (H₂S) and sulfur dioxide (SO₂) to elemental sulfur, as these species represent significant emission from pulping process. Various processes for the removal of SO₂ in the combustion gases are available. Direct catalytic oxidation of SO₂ to SO₃, and subsequent absorption of SO₃ in water to produce sulfuric acid, is an alternative method (Paik and Chung 1996). Total mill TRS emissions of 10–20% are contributed by vent streams from brownstock washers, foam tanks, black‐liquor filters, oxidation tanks, and storage tanks that are not typically collected in the noncondensable gas system (Pinkerton 1999). The emissions from these sources can be collected and combusted for energy recovery, reducing the atmospheric emissions of TRS and VOCs.

Freshwater Use Reduction and Chemical Recovery and Reuse Save Million at Pulp Mill

Effluent Discharges 

Before pulp mill effluents can be released to the environment they must be treated. Primary treatment involves the use of settling ponds or tanks in which suspended solids settle out of the liquid effluent. Solids can be composted and spread on land, converted to other useful products, or incinerated. Secondary effluent treatment includes oxidation and aeration in shallow basins having wide areas or in smaller areas using mechanical agitators and spargers to oxygenate fluids before release. Biological filter systems can be used to remove organic compounds and heavy metals, and often the process can be accelerated by adding nutrients and by using oxygen rather than air. In some areas, natural wetland systems have been designed to achieve this. Other means of treatment include lime coagulation and the elevation of pH to precipitate organic color bodies as calcium lignates. Once precipitated, the sludge is dewatered and incinerated to destroy organics.

It has been reported by NCASI that there was a 30% reduction in effluent flow from mills between 1975 and 1988. During the same period, final effluent 5‐day BOD and TSS decreased by 75 and 45% (NCASI 1991). Data for 1975, 1985, and 1988 are presented in Table 7.5. These reductions will continue as pulp and paper mills implement the so‐called best management plans (BMPs) required by the Cluster Rule. BMPs will require better management of process losses and spills and are expected to reduce effluent discharges. For example, in roughly 10 years, Simpson Tacoma kraft mill reduced its freshwater consumption by approximately 50% through various pollution prevention methods (K. Schumacher and M. Mammenga, personal communication; USEPA 1992a, b).

Table 7.5 Effluent discharges from pulp and paper mills.

Source: From Das (1997).

	1975	1985	1995
Effluent flow (gal/T)	22 800	17 200	16 000
BOD (lb/T)	18.0	4.8	4.4
TSS (lb/T)	13.0	8.3	7.1

Simpson Tacoma now uses about 18 million gal of freshwater per day (mgd) vs. 32 mgd in 1990s. This reduction in freshwater usage saves about $1.92 million/year (350/MG, city of Tacoma charges per average fixed and variable cost). The plant also saves money through reducing sodium hydroxide (NaOH) losses to sewers and to product fiber, by stopping overflowing weak washing dilute NaOH solution and through recycling processed water. Otherwise, NaOH would be needed to the process to make up for soda (sodium) lost. The saving is about 3.4 million/year (based on $375/T of NaOH and 25 T of NaOH saving per day). Simpson also saves $0.18 million/year by reducing losses of black liquor sulfur to sewer and stack (K. Schumacher and M. Mammenga, personal communication).

Other firms save water by installing savealls (devices which separate fiber from process water), heat exchangers, and other equipment which permit more reuse of process water. Internal water cleaning systems make it possible to substitute filtered white water for clean water. Separating process cooling and clean water is often necessary to achieve balance in operations and water use.

Brine Concentrator for Recycling Wastewater

Brine concentrators are vapor compression evaporator systems that produce distilled water and a very small salt concentrate stream. These are ideal for water recycling because the concentrate stream is so low that wastewater can be treated economically with a very high recovery and with no liquid discharge (Dalan 2005).

MINI‐CASE STUDY

SAVING THE COLORADO RIVER

The market for brine concentrators initially arose because of federal clean water regulations. The Colorado River, a major source of drinking and irrigation water for the southwestern United States, had been growing increasingly saline as a result of human activities. It was to control such damage to the environment here and elsewhere that the USEPA, in the 1970s, promulgated regulations curtailing discharges to the Colorado and forbidding construction of new plants that could not achieve Zero Discharge of water into the river.

To comply with the regulations, both new and existing power plants that were using river water had to recycle their water wastes. Although vapor compression was not new to industry as an energy source, the technological fit with power plants was a natural because it allowed these facilities to use electricity they were generating as the source of the mechanical energy needed in recycling water.

Federal law was not the only factor prompting industry to turn to the use of brine concentrators for recycling wastewater. All over the country, there were local siting regulations, as well. Thus, with the advent of the private power industry in the early 1990s, entrepreneurs turned to Zero Discharge water systems, which allowed them to use sites with a limited water supply, far away from discharge points. Similarly, the move to clean‐burning natural gas diminished the importance of locating power plants near sources of fuel or water.

As of 2004, there were approximately 60 brine concentrators in the Unites States. They are sold as package plants, designed and constructed with energy conservation principles. All the original units were at coal‐fired power plants, but as metal smelters, manufacturers of chemicals and semiconductors, and other enterprises began to recognize the need to eliminate water discharges, the use of brine concentration has spread.

Economics of Brine Concentrator Systems

Based on the parametric cost information on brine concentrator systems, we will now overview some economic aspects of these systems.

Brine concentrators, and by extension, integrated zero liquid discharge systems, only make sense in grassroots facilities only when

• there is a shortage of surface or well water
• the facility needs water to operate
• there is no water discharge option, such as a source of potable water or a river
• a discharge permit is unobtainable

Zero Liquid Discharge takes away the siting constraint; that is, it is no longer necessary to locate the plant near a large usable water source or suitable discharge point. The economic advantage of such an effect is hard to generalize, being very specific to the actual circumstance.

The NPV of a Zero Discharge facility is negative. Dalan and Rosain (1992) found that a 265 gpm brine concentrator had operating costs of $8.94/1000 gal of water treated (total of $1 252 600/year), while the avoided cost of extra demineralizer regeneration chemicals was $216 000/year. The avoided cost amounts to 1.54/1000 gal. Since any dollar amount here is an operating expense, the CF is negative.

Table 7.6 Operating cost breakdown for a 265 gpm system resulting in a cost of $9.62/1000 gal of feed.

Source: From Dalan and Rosain (1992), updated by Dalan (2005).

Item	Consumption	Unit cost ($)	Annual cost ($)
Operating labor	1 mhr (maximum hourly rate)/h	50/mhr	438 000
Maintenance (labor)	2 mhr/h	50/mhr	87 600
Maintenance (materials, including spare parts)			80 000
Electricity	1 617 kWh	0.05/kWh	707 000
Chemicals
Sulfuric acid	293 lb/day	0.06/lb	6 416
Polymers	40 lb/day	1.50/lb	21 900
Total chemicals		28 316
Total annual operating cost		1 340 916
$/1000 gal of feed		9.62

A typical specific operating expense is in the $5–7/1000 gal treated range. In this estimate is the price of electricity at a retail price of 0.05/kWh. In grassroots power plant planning, the electricity is many times considered a parasitic load on the power plant (electricity needed to produce the power). In this accounting method, the cost of electricity is zero. The elimination of electricity as an operating cost brings the total treatment cost down to the $2–3/1000 gal range. The typical energy load for a brine concentrator is 100 kWh/1000 gal of water produced.

The determination of water economics is very geography specific. For example, in western Washington, water and sewer bills typically are in the $1–2/1000 gal range, whereas in eastern Washington, water costs more, if indeed it is economically available.

For non‐power plant applications, local high prices for electricity can be overcome by seeking out alternate energy sources. Compressors (the majority energy user) in brine concentrator plants have been installed that are steam driven or natural gas (via a natural gas engine) driven. Table 7.6 presents breakdown of typical operating costs (Dalan 2005; Haussman and Rosain 1996).

For existing facilities, installing a Zero Discharge plant makes sense when

• the discharge permit conditions change, with the result that the existing facility can no longer be operated economically
• the cost of water rights plus treatment fees exceed the operating costs of a Zero Discharge facility

This last point is illustrated by a mini‐case study.

MINI‐CASE STUDY

CALCULATING PAYBACK FOR A ZERO DISCHARGE SYSTEM

A power plant has a 300 gpm blow downstream from the plant. Preliminary estimates show that a Zero Discharge plant would cost $4.55/1000 gal to operate. The local utility raises the water and disposal fees to $3.00/1000 gal, at a total cost to the plant of $6.00/1000 gal.

The capital cost of a 300 gpm of distillate flow rate is approximately $7.5 million (Dalan 2005). The annual saving is ($1.45 × 300 × 1 440 × 365)/1 000, or $228 636/year. This represents a project with a simple ROI of 3% and a simple payback of 32 years.