How to Size ZLD for Hydrostatic Test Water: 2026 Engineering Guide

Size a hydrostatic-test water recovery or zero liquid discharge (ZLD) system from the actual test batch, drain profile, treatment operating window, water chemistry and required reuse quality. First determine whether reuse or an authorized discharge route can meet the project requirements; a thermal ZLD train is a candidate when residual-liquid disposal must be eliminated. Low salinity alone does not establish that RO, evaporation or crystallization is necessary.

Define the operating basis before choosing equipment

Record the largest planned test volume, simultaneous tests, actual drain-rate profile, available storage, days between events and hours available for treatment. Average batch flow and instantaneous drain peaks are different. Storage may allow treatment equipment to run below the drain peak; its usable capacity must follow the event schedule and operating contingency.

Analyze representative water and every inhibitor or cleaning chemical used. Include conductivity/TDS, suspended solids, pH, temperature, oil, metals, hardness, silica and relevant inhibitor constituents. Define test-water reuse limits with the asset owner. A generic low-TDS description does not establish membrane compatibility or the required polishing process.

A consistent worked example: assumptions, not a project design

The following example assumes an 800 m³ batch, a constant 16-hour treatment window, 92% membrane water recovery, a trial permeate flux of 17.5 L/(m²·h), feed dissolved salt of 200 mg/L, negligible volume change, and ideal complete retention of dissolved salt in the concentrate. These inputs are selected solely to demonstrate arithmetic. In a real system some salt passes into permeate, chemicals add mass, solids may precipitate and cleaning produces additional streams.

The balance closes: 46 + 4 = 50 m³/h and 736 + 64 = 800 m³. The factor 0.001 converts m³·mg/L to kg. With the stated ideal assumption, the concentrate carries the original 160 kg; concentrating water does not create dissolved salt. A claimed concentration factor of 20 cannot be combined with 92% recovery and complete salt retention in this simple balance: its factor is 1 / 0.08 = 12.5.

The calculated area is only a starting arithmetic quantity. Select actual elements, staging, pressure, allowable recovery, temperature corrections and cleaning arrangements using the feed analysis and an appropriate membrane projection. The DuPont FilmTec technical manual describes flow and solute balances and the importance of operating within the designed recovery; it does not validate this example's assumed flux or 92% recovery for your water.

Separate concentrate flow from thermal energy

The example sends 4 m³/h of concentrate onward. This is a liquid feed rate, not a guaranteed evaporation rate: water remaining in solids, purge streams and operating interruptions affect the duty. A steam-consumption assumption cannot by itself establish MVR compressor electrical power.

Ask each thermal supplier for its own mass and energy balance at the measured feed composition, product moisture and temperature. Require clearly separated guarantees for electrical consumption in kWh per stated unit of feed or evaporated water, external steam consumption where used, start-up energy, cooling, turndown and cleaning. State the denominator and operating boundary for every figure. Do not convert a generic kg-steam/kg-water range into a compressor kW rating.

Specify the treatment train by function

Use storage and pretreatment to manage the actual solids and oil load, then evaluate membrane treatment only against a validated feed and reuse specification. Confirm the actual product and duty with the supplier before selecting equipment. Characterize membrane concentrate, pretreatment sludge and cleaning wastes separately; their handling and authorized disposal remain part of the project even when water is recycled.

EPA's industrial reuse resource explains that reuse requirements depend on the application and regulatory setting. Calling a system ZLD does not automatically remove permits, waste-management obligations or requirements for a separate stormwater or emergency-discharge stream.

RFQ and acceptance checklist

• Provide the event calendar, drain profile, storage operating limits and analytical results including additives.
• Request closed water and salt balances for normal operation, start-up, cleaning and exceptional streams.
• Specify usable treated-water quantity and quality for the test asset, concentrate/residue quantity and moisture, and the agreed sampling methods.
• Compare installed scope, storage, civil works, utilities, consumables, residual disposal and annual operating hours on the same basis.
• Agree the commissioning test duration, feed envelope, meter accuracy and procedure if the feed differs from the design basis.

There is no supported universal purchase price, payback period or annual test-frequency threshold in this example. Send the batch schedule, analysis and reuse target when requesting a project-specific quotation.