Food Waste Digestate Treatment

Client

Food Waste Digester Facility

Year

2020

Impact & Benefits

Recovered 87% of the digestate as UF permeate
Recovered about 90% of the UF permeate as clean water

Solutions

SWECO
Ultrafiltration (UF)
Two-Stage Reverse Osmosis™ (TSRO)

Case Study: Ultrafiltration, Food Waste Digester

A 100,000 GPD capacity Food Waste Digester facility form Michigan approached us to create clean water from their digester run-off. Digestate were first screened to remove large abrasive solids and fibers using a low-energy vibratory screen. Screened digested then filtered through our unique UF, designed to treat wastewater without the addition of polymers or processing aids, significantly reducing the operation cost.The UF permeate was of sufficient quality for sewer discharged.

Company Issue / Challenge

Manage Digester run-off, compliance with regulatory permit.

The cost for land application is high and runoff has few nutrients, so paying to truck this material to farm fields is not efficient.

Managing the storage and hauling of leachate is a constant challenge and requires coordination of various trucks throughout the year.

Concentrating the nutrients in the leachate for storage and later land application with manure would be most efficient.

What We Delivered

Site Assessment

We processed the raw digestate through an 18” diameter vibratory screen using a Market Grade 35 screen (447 microns with 37.9% open area). The screened digested then filtered through our unique UF. Our proprietary UF system uses stainless-steel modules containing multiple porous tubular UF membranes. For this project, we used a tubular membrane with an average pore size of 0.02 μm. We recovered about 87% of the digestate as UF permeate at a high average flux rate of ~40-10 GFD. The approach produced better quality water, while reducing the OPEX cost for the client.

Impact & Benefits

Our leachate filtration system is designed to recover more than 90% of the liquid as clean water for direct discharge.

Ultrafiltration

Recovered 87% of the digestate as UF permeate

Water Reclamation

Recovered about 90% of the UF permeate as clean water

Analysis of Ultrafiltration Process Streams

PARAMETER	Unit	UF Feed	UF Permeate	UF Concentrate	Percent Removed by UF
Total Solids	mg/L	15,900	5,700	52,604	64%
Biochemical oxygen demand (BOD)	mg/L	673	<149.2	NA	>78%
Chemical oxygen demand (COD)	mg/L	16,460	800	41,900	95%
Ammoniacal Nitrogen	mg/L	1,020	971	1,210	5%
Organic Nitrogen	mg/L	650	39	3,439	94%
Total Kjeldahl nitrogen (TKN)	mg/L	1,670	1,010	4,661	40%
Phosphorus	mg/L	281	12.2	1,500	96%
Potassium	mg/L	1,010	953	1,300	6%
Sulfur	mg/L	84	25.2	407	70%
Calcium	mg/L	441	141	1,821	68%
Magnesium	mg/L	75	52.6	203	30%
Sodium	mg/L	1,130	1,120	1,318	1%
Iron	mg/L	33.2	0.23	196	99%
Manganese	mg/L	0.793	0.071	4	91%
Zinc	mg/L	1.66	n.d.	9	100%
Copper	mg/L	0.42	n.d.	2	100%
Conductivity	ms/cm	14.1	13.9	NA	n/a
pH	S.U.	7.87	8.04	8	n/a

n/a: not applicable; n.d.: not detected , NA: not analyzed

As expected, the UF removed the vast majority of the total solids, organic nitrogen, and phosphorus from the digested food waste sample, producing a transparent permeate. The COD reduction was 95% and the BOD reduction was greater than 78%.

Analysis of Two-Step Reverse Osmosis Process Streams

PARAMETER	Unit	TSRO Feed	TSRO Permeate	TSRO Concentrate	Percent Removed by TSRO
Total Solids	mg/L	5,700	200	28,100	96%
Chemical oxygen demand (COD)	mg/L	800	22	5,400	97%
Ammoniacal Nitrogen	mg/L	971	10.5	1,170	99%
Organic Nitrogen	mg/L	39	0.2	60	99%
Total Kjeldahl nitrogen (TKN)	mg/L	1,010	10.7	1,230	99%
Phosphorus	mg/L	12.2	n.d.	9.69	100%
Potassium	mg/L	953	2.7	1,650	100%
Sulfur	mg/L	25.2	n.d.	74.4	100%
Calcium	mg/L	141	n.d.	113	100%
Magnesium	mg/L	52.6	n.d.	164	100%
Sodium	mg/L	1,120	114	8,280	90%
Iron	mg/L	0.23	n.d.	0.38	100%
Manganese	mg/L	0.071	n.d.	0.157	100%
Zinc	mg/L	n.d.	n.d.	0.05	100%
Copper	mg/L	n.d.	n.d.	0.1	100%
Conductivity	ms/cm	13.9	0.71	39.3	n/a
pH	S.U.	8.04	5.71	8.27	n/a

n/a: not applicable; n.d.: not detected , NA: not analyzed

These data indicate that the TSRO produced very clean water with very low levels of N, P, and K. Only 10.1 mg/L of ammonia and 17.2 mg/L of total nitrogen was detected in the RO permeate. The high levels of sodium in the concentrate are an artifact of batch concentration on our pilot unit and is not expected commercially.