MEMORANDUM

TO: Lisa Biddle, EPA/EAD 

 

FROM: Brent Ruminski, ERG 

DATE: 7 March 2014

SUBJECT: Notes on Call with Hydrozonix, LLC on 7 February 2014 

On 7 February 2014, ERG held a meeting with representatives from Hydrozonix, LLC 

(Hydrozonix), a wastewater treatment vendor that offers advanced oxidation precipitation (AOP) 

treatment systems to unconventional oil and gas (UOG) operators. Attachment 1 lists all of the 

meeting participants. The main discussion topics included: 

 Overview of UOG wastewater treatment for reuse 

 How AOP works 

 How operators use AOP 

 Targeted pollutants and removals 

 Costs and energy requirements 

Overview of UOG Wastewater Treatment for Reuse 

Since 2010, Hydrozonix has provided wastewater treatment services, on a contract basis, 

to UOG operators for reusing the treated wastewater in hydraulic fracturing. To date, they have 

treated over 37 million barrels of UOG wastewater originating from the following UOG 

formations. The approximate number of wells hydraulically fractured with wastewater treated by 

Hydrozonix’s AOP treatment unit is also noted below. 

 Eagle Ford (shale oil and gas): 7 wells 

 Fayetteville (shale gas): 10 wells 

 Marcellus (shale gas): 9 wells 

 Wolfcamp/Spraberry (shale/tight oil and gas): 430 wells 

The use of AOP for UOG wastewater treatment enables operators to reuse wastewater in 

hydraulic fracturing. Hydrozonix reported that the potential for operators to reuse wastewater 

ultimately depends on the total cost compared to other management alternatives. The total cost 

for managing and/or disposing of UOG wastewater depends on many factors, including the 

following: 

 Drilling/completion program1

 

 

1

 Operators must have an active drilling and completion program in order to have a successful reuse program. 
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 Wastewater volumes 

 Wastewater quality 

 Fracturing fluid design (e.g., gel, slickwater) 

Hydrozonix has served customers that hydraulically fracture their wells using both “gel” 

and “slickwater” fracturing fluid designs. The remainder of this discussion focuses on the status 

and progress of reuse activities in the UOG industry in terms of UOG wastewater quality and 

fracturing fluid design (gel versus slickwater). 

Gel Designs

Operators typically use gel designs to fracture liquid rich formations2

. Gel designs are 

water-based fracturing fluids and that contain gel chemical additives (e.g., guar gum). Gel 

designs have relatively high fluid viscosity compared to slickwater designs, which increases the 

quantity of proppant that can be carried into the formation fractures. Hydrozonix indicated that 

gel designs are more complex than slickwater designs and therefore present more obstacles for 

reuse than slickwater designs. In general, gel designs are not tolerant to base fluids3

 with 

elevated TDS concentrations. For example, Hydrozonix indicated that most fracturing service 

providers have a maximum TDS concentration for base fluid between than 20,000 to 50,000 

mg/L for gel designs. 

Hydrozonix provided several specific examples of issues with using high TDS base fluid 

for gel fracturing: 

 Gel fracturing fluid designs typically use “delayed” cross-linked gels that allow the 

properties of the gel to activate after injection. Boron is an example of a constituent that 

is present in UOG wastewater that may cause these gels to activate prematurely. High 

levels of TDS in the base fluid can also cause incompatibility issue for the gel. 

 Gel designs typically require the service provider to increase the pH above neutral in 

order for the chemistry to work properly. This pH adjustment can cause constituents in 

UOG wastewater to precipitate and cause formation damage. 

Hydrozonix suggested that some of the service providers recently began testing new 

chemical additives that allow the use of base fluids with higher TDS concentrations for gel 

designs. However, Hydrozonix expects these designs to be expensive initially for operators to 

use compared to existing designs. 

Slickwater Designs

Operators typically use slickwater-fracturing fluids in dry natural gas producing 

formations. Slickwater designs are water-based fracturing fluids where primary purpose of the 

chemical additives is to reduce friction (i.e., friction reducers). In the past, fracturing service 

providers had concerns about high concentrations of TDS in base fluids because they could 

interfere with friction reducers. However, chemical additives have advanced and today, operators 

 

2 Liquid rich formations are those that produce oil or natural gas co-produced with natural gas condensates.

3

 Base fluid is the final mixture of source waters to which chemicals and proppant are added to create the fracturing 

fluid. Base fluid can be only freshwater, or a mixture of freshwater, groundwater, and reused UOG wastewater. 
Page 3 of 9

can use base fluids with relatively high TDS concentrations compared to gel designs. 

Hydrozonix stated that base fluids with TDS concentrations as high as 100,000 mg/L are not 

uncommon in slickwater designs and that some operators do not have a maximum TDS criteria 

anymore. 

How AOP Works 

AOP creates ozone and hydroxyl radicals to destabilize targeted constituents such that 

they precipitate out of the solution. AOP does not require chemical additives. Instead, AOP uses 

a series of processes that hydrolyze water into hydrogen and hydroxyl radicals. Hydroxyl 

radicals are very reactive and are ideal for destabilizing targeted constituents. The following 

sequence of processes occurs within an AOP treatment system: 

1. Wastewater enters static mixers that homogenize the wastewater and initiate 

hydrodynamic cavitation using a static mixer. Within the static mixer, the velocity 

of the wastewater is increased and the pressure is simultaneously decreased, 

resulting in the creation of small vapor cavities (i.e., bubbles). These cavities 

quickly collapse and release a sufficient amount of energy to cause water 

molecules to break down into hydroxyl radicals. Cavitation also serves as a 

catalyst for chemical interaction by increasing the surface area between 

constituents within the water (i.e., mixing). 

2. Ozone (O3)

4 , created by oxygen separators and ozone generators, is mixed with 

the wastewater in a flash reactor. The ozone acts as an oxidizer and immediately 

kills bacteria. Only enough ozone is injected so that all of it is consumed before 

the wastewater leaves the treatment unit. Excess ozone can potentially interfere 

with fracturing fluid chemicals. As currently configured, AOP does not include an 

ozone recovery and destruction step. Instead, the treatment unit operator 

determines the appropriate dose of ozone using real-time monitoring of oxidation 

reduction potential (ORP)5 in the UOG wastewater. 

3. The wastewater passes through an electrochemical oxidation unit. 

Electrochemical oxidation uses electrical current to convert water molecules into 

hydroxyl radicals. These hydroxyl radicals aid in destabilizing suspended, 

emulsified, or dissolved contaminants. 

4. The cavitated, ozone-rich wastewater is introduced to ultrasonic transducers 

which generate an oscillating pressure wave (i.e., sound). These high pressure 

waves provide sufficient energy to initiate caustic cavitation. Caustic cavitation is 

the creation of small vapor cavities using pressure waves. These vapor cavities 

immediately collapse, resulting in a release of energy that causes additional 

breakdown of water molecules into hydroxyl radicals. 

 

4

 This ozone is generated within the treatment system. Ozone is a highly reactive oxidant that kills bacteria and 

oxidizes heavy metals. 

5

 ORP is the voltage required for chemical species to acquire electrons (i.e., reduced). 
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5. The ultrasonic transducers simultaneously reduce the particle size of destabilized 

constituents to less than one micron. 

During AOP, the precipitated solids in the wastewater are intentionally not removed 

during AOP. Instead, the size of the precipitated particles is reduced to less than one micron so 

they do not cause formation damage during fracturing. In addition, they no longer have the 

potential to cause scale during the hydraulic fracturing process because they are destabilized. 

How UOG Operators Use AOP 

Operators most commonly use AOP to treat UOG wastewater on-the-fly with hydraulic 

fracturing as shown in Figure 16

. On-the-fly treatment is only possible because mobile AOP units 

are capable of treating wastewater at relatively high rates compared to other technologies.7 In onthe-fly treatment, other source water (e.g., fresh water, ground water) and the untreated 

wastewaters are mixed together and treated by the AOP treatment unit. Although not shown in 

Figure 1, AOP typically requires media filtration to remove suspended solids prior to treatment. 

Either Hydrozonix or the UOG operator provides filter bags that are used immediately before the 

primary AOP treatment unit. 

On-the-fly AOP has the following potential cost savings to the operator: 

 Elimination of trucking wastewater to a centralized facility for treatment 

 Reduction or elimination of the use of biocides and scale inhibitors during fracturing 

 Storage of the untreated and treated wastewater because it is already required for 

hydraulic fracturing8

 

Occasionally, operators may use AOP in an alternative setup shown in Figure 2. In this 

set up, AOP treatment units are deployed at existing impoundments that UOG operators 

construct to temporarily store wastewater.9 The AOP treatment unit circulates wastewater back 

into the impoundment after treatment. Because AOP can treat wastewater at relatively high rates, 

it is possible to treat the entire volume of a typical impoundment in one day, although multiple 

passes of treatment are typically necessary to guarantee sufficient treatment.10 Operators that 

choose to implement AOP treatment at an existing impoundment typically do not need to 

purchase any additional equipment other than the rented unit. The use of AOP at existing 

 

6

 UOG operators may store untreated wastewater in open pits or impoundments adjacent to well pad depending on 

applicable regulations. 

7

 Treatment rates of 80 barrels per minute are typical for AOP treatment, but rates can be as high as 120 barrels per 

minute. Higher treatment rates result in reduced treatment effectiveness. For comparison, a typical fracturing job 

will inject fracturing fluid at approximately 60 to 120 barrels per minute. If needed, operators can use a second AOP 

system in parallel to meet the desired fracturing rate. 

8

 If treatment prior to reuse is conducted at a centralized location, which is typical for other treatment technologies, 

untreated and treated wastewater requires storage at that facility in addition to its own storage at the next fracturing 

site. 

9

 Applicable regulations for storing UOG wastewater in open impoundments vary by state and sometimes even 

within states. 

10 Based on a typical impoundment capacity of 100,000 barrels and a treatment rate of 60 to 80 barrels per minute. 
Page 5 of 9

impoundments has many of the same advantages as the use of on-the-fly AOP treatment, as 

explained above. One additional advantage of AOP treatment at an existing impoundment is that 

the AOP can operate continuously, whereas on-the-fly treatment requires periodic breaks while 

the operator moves onto the next stage of the fracturing job. 

Figure 1. Flow Diagram of AOP Treatment On-the-Fly with Hydraulic Fracturing in UOG 

Applications 

Figure 2. Flow Diagram of AOP Treatment at Wastewater Impoundments in UOG 

Applications 
Page 6 of 9

Targeted Pollutants and Removals 

Operators use AOP for treatment prior to reuse because it focuses on microbial control 

and scale control, two issues that UOG operators must avoid. Microbial constituents are typically 

totally reduced by AOP treatment. During AOP treatment, the multivalent cations that typically 

cause scale in UOG wells are destabilized and then reduced in size to less than 1 micron and 

remain suspended within the solution. For the purpose of reuse in fracturing, UOG operators 

often only require this level of treatment because the scaling potential is reduced. 

Table 1 shows specific influent and effluent concentrations from AOP treatment of UOG 

wastewater. Data in Table 1 confirm that the presence of bacteria decreases significantly, but 

concentrations of many multivalent cations are not. However, Figure 3 demonstrates how AOP is 

able to reduce scaling potential. This figure shows the results from dynamic tube block tests11 of 

treated UOG wastewater (red) and the associated untreated UOG wastewater (blue). The increase 

in pressure observed for the untreated UOG wastewater indicates its higher potential to cause 

scale formation than the treated UOG wastewater. 

Table 1. AOP Removal Efficiency in UOG Wastewater Applications 

Constituent 

Typical Influent Range 

(mg/L, unless otherwise 

noted) 

Typical Effluent Range 

(mg/L, unless otherwise 

noted) 

Removal Efficiency 

(%)

Chlorides 17,650 – 19,250 17,250 – 19,200 0 – 2 

Metals 

Barium 56 – 285 42 – 250 12 – 25 

Calcium 427 – 2,408 323 – 2,360 2 – 24 

Iron, total 21.2 – 144 19.7 – 143 1 – 7 

Iron 15.7 – 147 5.9 – 148 1 – 62 

Magnesium 317 – 736 290 – 680 8 – 9 

Other 

Carbonate 4.5 – 92 2.8 – 58 37 – 38 

Alkalinity 150 – 700 95 – 530 24 – 37 

Calcium hardness 1,840 – 6,020 1,700 – 5,900 3 – 5 

Hardness 2,050 – 7,340 1,950 – 7,110 2 – 8 

Bacteria (counts/mL) 230 – 9,300 BRL – 4 > 99% 

Sulfates 100 – 855 85 – 657 15 – 23 

BRL: Below Reporting Limit 

Source: As reported by Hydrozonix in Appendix B. Hydrozonix reported bacteria data separately. 

 

11 A dynamic tube block test is used by UOG operators to measure the scaling potential of a fluid. Operators use this 

test to determine the efficiency of chemical inhibitors that prevent scaling in the wellbore. This test measures the 

pressure change over time as a fluid (e.g., UOG wastewater) flows through a tube apparatus. An increase in pressure 

indicates the buildup of scale within the apparatus. NACE International, The Corrosion Society, reports specific 

testing protocols in NACE 31105, Dynamic Scale Inhibitor Evaluation apparatus and Procedures in Oil and Gas 

Production. 
Page 7 of 9

Figure 3. Test Results of a Dynamic Tube Block Test after AOP Treatment 

Costs and Energy Requirements 

As previously mentioned, Hydrozonix offers their treatment system to operators on a 

contract basis. The typical cost charged to operators ranges from $0.50 to $0.75 per barrel of 

influent wastewater12. Higher costs are possible depending on set up costs and excessive 

downtime between fracturing stages. Costs are minimized when downtime between fracturing 

stages is minimized or the treatment is performed at an impoundment (see Figure 2). 

Hydrozonix’s treatment unit typical runs on electrical energy supplied by an onsite 

generator. Hydrozonix estimated that their technology requires between 1.9 and 3.6 kilowatt 

hours per 1,000 gallons of influent wastewater using. 

 

12 This includes any other source water (e.g., freshwater, groundwater) that is blended with the untreated UOG 

wastewater prior to treatment. 
Attachment 1: List of Meeting Participants

Name Affiliation

Jinxuan Hu, Engineering Manager Hydrozonix, LLC 

Mark Patton, Director of Marketing Hydrozonix, LLC 

Brent Ruminski, Engineer Eastern Research Group, Inc. 

 
Page 9 of 9

Attachment 2: General Water Qualities for Untreated Water and Treated Water 

Provided by Hydrozonix